How Do a Cold, the Flu, and Pneumonia Differ? — An Infection That Stays in the Entryway, and One That Reaches the Inner Room

"I Have a Cough and a Fever, Too"

My throat felt scratchy since the morning, and by evening, a cough started and a fever rose. Someone says it's a cold, someone asks if it might be the flu, and someone tells me to go to the hospital because if it lasts a long time, it could be pneumonia. I also remember getting antibiotics from the hospital during my last cold.

All three names refer to illnesses caused by germs entering the breathing passage. Two questions separate the three. How far inside the body the germs have penetrated, and what has entered.

This article follows the two questions in order to look at how the three differ, where antibiotics work and where they do not, and what kind of appearance should prompt you to see a doctor promptly.

The House Called the Airway, and Two Groups That Enter

The airway is the passageway through which air travels in and out from the nose and mouth deep into the lungs. At the end of it lie the lungs. The lungs are organs situated one on each side inside the chest, where oxygen and carbon dioxide are exchanged between the inhaled air and the blood.

Air that has passed through the nose and throat travels down a thick tube into the chest. The bronchi are the tubes where the airway descending from the neck branches into both lungs and gradually narrows. At the end of the branches hang air sacs. The alveoli are tiny air sacs hanging like grape clusters at the end of the airway, where oxygen and carbon dioxide move back and forth with the blood.

We compare this pathway to a house. The nose and throat are the entryway where outside air first touches, the thick airway below the neck and the bronchi are the hallway leading inward, and the alveoli are the deepest inner room.

The first question in dividing infections is how far into this house they have come. Upper respiratory tract infections are infections that occur in the upper part of the airway, such as the nose and throat. Lower respiratory tract infections are infections that occur in the bronchi and lungs below the neck. Lower respiratory tract infections is a collective name for several diseases ranging from bronchial infections to lung infections, and these diseases differ in both the magnitude of their danger and how they are handled (PMID 36261157).

Infections that have reached the inner room have a separate name. Pneumonia is an infection that occurs in the alveoli and the fine airways near them.

The second question is what has entered. The things that cause infections in the airway are mainly two groups. Viruses are tiny pathogens that cannot multiply on their own and must enter the cells of other living things to multiply. Bacteria are small living things made of a single cell, some of which cause disease.

The reason for dividing these two groups lies in medicine. Antibiotics are drugs that kill bacteria or stop them from growing, and they do not work against viruses. The house drawing only indicates where the infection has settled. What has entered cannot be known from the drawing alone, and the same group can enter the entryway or the inner room.

Infections That Stay in the Entryway — The Common Cold

Infections of the nose and throat are the most common infections in children, and the primary reason antibiotics are prescribed for them. These infections are mostly caused by, or start with, a virus, and even when bacteria are present, they usually resolve on their own (PMID 37730165).

A cold is a mild viral infection of the upper airway, and usually gets better on its own. It is estimated that viruses cause around 90% of upper airway infections (PMID 12029606). Children catch up to five colds a year, and adults two to three, with symptoms usually settling within a week, though a cough can linger longer (PMID 21406124).

Some of the symptoms experienced with colds and the flu—such as fever, loss of appetite, fatigue, chills, headaches, and body aches—are explained by the action of signaling molecules released by the body (PMID 16253889).

Most infections that enter the entryway are cleared by the body itself. This fact is the starting point for the story about antibiotics that we will see later.

A Cough That Reaches the Hallway — Acute Bronchitis

The most common reason people visit outpatient clinics for illness in the United States is a cough (PMID 27929206). The diagnosis people who visit primary care for a cough usually receive is an infection of the hallway (PMID 21121518).

Inflammation is a response in which the body fights back against stimuli like damage, infection, or substances from outside the body by calling in immune cells, causing the area to swell, and reacting. Acute bronchitis is a condition where inflammation suddenly develops in the large airways below the throat, leading to a lingering cough, with no signs of pneumonia. Viruses cause more than 90% of acute bronchitis cases (PMID 21121518). It is usually a condition that resolves on its own (PMID 40106287).

This cough lasts a long time. The cough of acute bronchitis commonly goes on for about 2 to 3 weeks (PMID 27929206). Even if the phlegm takes on a color like green, that color cannot reliably distinguish between a bacterial infection and a viral infection (PMID 21121518). This means that the mere fact that a cough has continued into its second week and the phlegm has grown darker cannot be read as the illness having shifted toward bacteria.

When looking at a hallway cough, the first thing a doctor tries to sort out is whether that cough comes from pneumonia. In the cough of an adult without an underlying disease or hidden asthma, this is the primary goal of the evaluation (PMID 11119400). However, if the cough continues for more than 2 weeks and comes in paroxysms, if someone vomits at the end of a cough, or if there is a history of close contact with a patient with pertussis, another bacterial infection called pertussis is suspected (PMID 27929206).

A Named Virus — Influenza

Influenza is an airway infection caused by influenza viruses, commonly called the flu. Colds and influenza are caused by different viruses. Influenza viruses include type A and type B, and type A is further divided into subtypes such as H1N1 and H3N2 (PMID 39778922).

These viruses change their outer appearance easily. That is why vaccines must be newly designed every year to match the subtypes circulating at the time (PMID 39778922). Influenza is also a disease that appears on the list of lower airway infections (PMID 36261157).

The magnitude of the risk varies greatly depending on the person. In U.S. insurance claims data, among about 1.6 million influenza patients who visited medical institutions during the 2014–2018 influenza season, 1.2% were hospitalized. In the same data, the odds of being hospitalized were calculated to be roughly 9 times higher for people aged 65 and older than for those aged 5 to 17, and 2 to 3 times higher for certain underlying medical conditions (PMID 36171601). This multiple was measured using odds—a way of comparing the likelihood of being hospitalized—and came from calculations that factored in conditions like age and underlying diseases all at once. Most influenza virus infections end mildly on their own, and only some become severe enough to require hospitalization, with the weight of the disease increasing at both ends of the age spectrum and in the presence of underlying illnesses (PMID 32988982). This means that while most people who receive medical care pass through without hospitalization, the risk multiplies severalfold for older adults and those with underlying conditions. Viral pneumonia such as influenza also tends to be more severe in pregnant women (PMID 39893062).

A complication is a disease that follows another disease in another part of the body. There are specific drugs targeted at influenza. Antiviral drugs are medications that prevent viruses from multiplying inside the body. The American Academy of Pediatrics (AAP) recommendation for the 2025–2026 season advises antiviral treatment for children with suspected or confirmed influenza who are hospitalized, have severe or worsening illness, or have underlying conditions that increase the risk of complications, and states that treatment should be started as early as possible regardless of the duration of illness. For other children, it leaves the option to be chosen in an outpatient setting depending on the circumstances (PMID 40717223). One review addressing adult pneumonia recommends testing all pneumonia patients for both influenza and COVID-19 when influenza and COVID-19 are circulating. This is because antiviral treatment and infection management differ depending on the results (PMID 39283629).

There is also vaccination. The same American Academy of Pediatrics (AAP) recommendation advises annual influenza vaccination for all children aged 6 months and older who have no contraindications (PMID 40717223). On the adult side, one review notes that the benefits of vaccination are receiving increasing attention for pregnant women, older adults, and people with underlying conditions who are vulnerable to influenza complications (PMID 32109627).

Vaccine effectiveness varied by season, age, and subtype. In an analysis pooling 191 studies published between 2017 and 2022, vaccines reduced test-confirmed influenza by roughly 41%, and that figure was around 49% in those under 18 years old, around 31% in those 65 and older, and lower at around 27% for the H3N2 subtype (PMID 38423813). In an analysis pooling 165 studies of hospitalized individuals, it was estimated that influenza-related hospitalizations were reduced by 42% and influenza-related pneumonia by 51%, with effectiveness being greater in children and during seasons when the vaccine and the circulating virus matched well (PMID 41093140).

Certainty of evidence is a grade—rated as high, moderate, low, or very low—indicating how much we can trust the pooled results. The latter analysis rated the certainty of the effect in reducing hospitalization as "very low," and the certainty of the effect in reducing influenza-related pneumonia as "low" (PMID 41093140). The studies pooled in both analyses are studies that compared vaccinated and unvaccinated individuals based on test results in actual clinical practice. This percentage is a relative reduction compared to unvaccinated people, and how many fewer people actually fall ill depends on how widely influenza circulated that season.

An Infection That Reaches the Living Room — Pneumonia

In the United States, community-acquired pneumonia leads to roughly 1.4 million emergency department visits, 740,000 hospitalizations, and 41,000 deaths each year (PMID 39283629).

Pneumonia is a common acute airway infection, and across all ages, it is a major health problem that can lead to short or long-term death. Pneumonia is broadly divided into community-acquired and hospital-acquired types (PMID 33833230). Community-acquired pneumonia is pneumonia contracted during daily life outside of a hospital.

Many things find their way into the living room. Pneumonia is caused by various microorganisms, including bacteria, airway viruses, and fungi, and which one is common varies greatly by region (PMID 33833230). Often, the cause remains unknown to the end. Among patients hospitalized with community-acquired pneumonia, the causative pathogen is identified in only 38%, and among those identified, viruses are cited as the cause in as many as 40% (PMID 39283629). In children's pneumonia, viruses are more common, with respiratory syncytial virus (RSV) being the most frequent cause under the age of two (PMID 34913645). In children's pneumonia, reducing the overuse of antibiotics and antibiotic resistance are cited as urgent tasks (PMID 38112800).

There are people whom pneumonia visits more frequently and more severely. It is more common in children under five and older adults with chronic illnesses (PMID 33833230). People aged 65 and older, those with chronic lung disease, smokers, and those with suppressed immunity are at the greatest risk for community-acquired pneumonia and its complications, which include a condition where an infection shakes organs throughout the body, a condition where the lungs are severely damaged and breathing collapses, and death (PMID 39283629).

Diagnosis looks at body signals and images together. Community-acquired pneumonia is diagnosed when there are two or more symptoms such as a new or worsening cough or shortness of breath, or signs like fever, accompanied by matching chest imaging findings, with no other explanation (PMID 39283629). A chest X-ray is a test that uses X-rays to look at the chest and see the appearance of the lungs. In very elderly people, pneumonia appears atypical and vague, leading to frequent misdiagnoses of pneumonia when it is something else, and chest X-rays are often of poor quality or inconclusive, so CT or ultrasound sometimes helps confirm the diagnosis (PMID 40021428).

The pillar of community-acquired pneumonia treatment is antibiotics (PMID 41110447). Since the cause is often unknown at first, doctors promptly start antibiotics targeting the most plausible bacteria before the cause is identified (PMID 30955516, PMID 39283629). Different types of antibiotics work against different bacteria, and some antibiotics do not work against bacteria that lack a cell wall (PMID 34750083). Pneumonia was once considered a disease that ends acutely, but it is now viewed as a condition that can leave long tails such as cardiovascular events, decreased breathing function, and cognitive decline. Comprehensive care for pneumonia includes rehabilitation after the acute phase, cardiovascular checks, and prevention such as vaccination (PMID 41110447). In children, vaccines targeting two main bacteria that caused pneumonia have significantly reduced pneumonia caused by those bacteria over the past 30 years (PMID 38112800).

There is more than one way to enter the living room. Pneumonia connected to things from the mouth and throat passing down into the airway is covered separately in "Is Aspiration Pneumonia Really Caused by Aspiration?". Tuberculosis, which is airborne, is taken over by another part of this series, the one covering tuberculosis.

Where Antibiotics Work and Where They Don't

When records of outpatient visits for respiratory tract infections were compiled, antibiotics were prescribed in 66% of cases. Studies judged 45% of these prescriptions to be inappropriate—meaning they were used without need, prescribed in insufficient amounts, or involved picking a drug that went against guidelines—and the most common reason was using them when there was no reason to. The illnesses that most frequently triggered these prescriptions were acute bronchitis, throat inflammation, inflammation of the spaces inside the nose, and the common cold (PMID 39243046). This is the result of pooling 36 studies from various countries.

Let us look at the common cold first. A Cochrane Review is a type of literature synthesis that gathers and analyzes studies using a set procedure, and is re-issued with a revised framework when new studies accumulate. A placebo is a fake medicine that looks and is used just like a real drug, but has no active medicinal ingredients. In a Cochrane Review pooling trials up to 2013, people with colds that started less than a week ago who received antibiotics did not get better any faster, nor did they differ in the rate at which symptoms lingered, compared to those who received a placebo (six trials). Side effects were 1.8 times more frequent in the antibiotic group, and 2.6 times more frequent in adults, while children did not show an increase in side effects. Even when pus-like nasal discharge persisted, there was no evidence that antibiotics provided any benefit, and side effects increased regardless of age (PMID 41277585).

The conclusion of this review is that there is no evidence antibiotics provide any benefit for colds in either children or adults, that there is evidence antibiotics cause meaningful side effects when used for colds in adults, and that routine use of antibiotics for the common cold is therefore not recommended (PMID 41277585). Antibiotics have no evidence of curing colds caused by viruses, and they add side effects for adults.

In acute bronchitis, antibiotics do not help the illness get better overall, and while they can shorten a cough by about half a day, they bring side effects (PMID 40106287). Those side effects include allergic reactions, nausea and vomiting, and a severe diarrheal infection in the gut (PMID 27929206). A Cochrane Review from 2000 concluded that the magnitude of that benefit is similar to the harm that side effects can cause (PMID 11034678). Review articles also note that antibiotics are not necessary for acute bronchitis in people without chronic lung disease (PMID 27929206). However, antibiotics are sometimes used when pertussis is suspected, or for people at high risk of progressing to pneumonia, such as those aged 65 and older (PMID 21121518).

Clinical guidelines point in the same direction. In 2016, the American College of Physicians (ACP) and the Centers for Disease Control and Prevention (CDC) advised that for healthy adults without chronic lung disease or immune suppression, doctors should not prescribe antibiotics for the common cold, and should not initiate tests or antibiotics for bronchitis unless pneumonia is suspected (PMID 26785402).

The same advice separately outlines the conditions under which antibiotics should be used. These are cases where a specific bacterial throat infection is confirmed by a test, and cases of inflammation of the spaces inside the nose where symptoms last longer than 10 days, high fever and pus-like nasal discharge continue for at least three days in a row, or symptoms that were improving start getting worse again (PMID 26785402). For pneumonia, antibiotics are the pillar of treatment (PMID 41110447). There are definitely respiratory infections that require antibiotics, and distinguishing those situations is a matter of the doctor's physical examination and testing.

Unnecessary prescriptions also carry other costs. Antibiotic resistance is when bacteria develop the ability to withstand antibiotics, making those drugs stop working well. In various studies comparing national-level antibiotic use and resistance, countries that used antibiotics more heavily showed increased resistance in the bacterial groups that commonly cause respiratory infections (PMID 19366364). Resistance makes infections more severe, prolongs hospital stays, and increases deaths (PMID 25436105).

What drives prescriptions in the clinic? In an analysis of 28 studies on antibiotic prescriptions for respiratory infections, whether patients actually wanted antibiotics was either not linked or only weakly linked to prescriptions, whereas whether doctors believed patients wanted antibiotics was linked much more strongly. The research team therefore recommends that patients and doctors emphasize clear and direct communication (PMID 27139474). Delayed prescription—where a doctor gives a prescription but tells the patient to wait a few days and use it only if they do not get better—reduced antibiotic use significantly compared to immediate prescription, and did not differ from no prescription in terms of symptom control and complications (PMID 37791590).

The Same Cough and Fever, Three Diverging Roads

Now we place a person's cough and fever onto the house illustration. That person cannot know by themselves in which room the infection has settled. That is why the doctor asks, looks, and measures.

The first thing the doctor checks for is the hint that it has reached the master bedroom. If breathing is fast, the pulse is weak, breath is short, or sounds suggesting pneumonia are heard when listening to the chest with a stethoscope, pneumonia is suspected and a chest X-ray is taken (PMID 27929206). When examining an adult without underlying diseases or hidden asthma, if the pulse rate, breathing rate, and oral temperature measured by the doctor are all normal, the probability of pneumonia is considered very low (PMID 11119400). In very elderly people, pneumonia can present in atypical ways, so this judgment does not hold up well (PMID 40021428).

If there is no such hint, the probability of pneumonia is low, and the doctor thinks of infections in the entryway or hallway along with other causes such as asthma, exacerbation of chronic lung disease, pertussis, or influenza (PMID 40106287). If it is a hallway cough, lasting about 2 to 3 weeks is a common clinical course (PMID 27929206), and antibiotics can only shorten the cough by about half a day while inviting side effects (PMID 40106287). If findings matching pneumonia are seen on the chest X-ray taken because of the hints and there is no other explanation, the infection is diagnosed as pneumonia settled in the master bedroom (PMID 39283629). If it is pneumonia, antibiotics become the pillar of treatment (PMID 41110447).

This road also comes with the exceptions we saw earlier. Darkened sputum color cannot distinguish between bacteria and viruses (PMID 21121518). When a doctor suspects pneumonia in a child, the findings most strongly connected to pneumonia on a chest X-ray were grunting breathing sounds, a history of fever, chest retractions when breathing, crackling sounds heard through a stethoscope, fast breathing, and the overall impression received by the doctor (PMID 34913645). Even so, in childhood pneumonia, combining physical exams, blood tests, and imaging still does not reliably distinguish whether it is due to bacteria or viruses (PMID 38112800). If it is pneumonia, when influenza is circulating, a viral test sorts out that specific name (PMID 39283629).

The junction where the same cough and fever split into a cold, acute bronchitis, and pneumonia is thus created by the doctor's questions, physical examination, and tests.

Signs That Require Haste

Running out of breath to the extent that it is hard to finish a sentence, lips turning blue, clouding of consciousness, or becoming hard to wake are things that medical guidelines list as danger signs that require immediate medical attention. Such signals are a reason to seek medical care promptly on their own, without waiting to know the name of any disease.

Among them, a clouded consciousness and accelerated breathing are items included in scoring systems (such as CURB-65) that gauge the risk of death in pneumonia patients (PMID 27960212, PMID 34574976). Pneumonia patients who need intensive care are handled as emergencies, and in older patients, a low score does not mean the risk is low (PMID 27960212).

In children, if blood oxygen is lacking or breathing looks extremely difficult, a chest X-ray is taken, and it is considered that home treatment is possible only when such labored breathing is absent (PMID 34913645). In people of very advanced age, pneumonia may appear in a hazy and inconsistent manner (PMID 40021428).

There are also people who are at particularly high risk. People aged 65 or older, people with chronic lung diseases, smokers, and people with suppressed immunity are at the greatest risk for community-acquired pneumonia and its complications (PMID 39283629), and older people and those with chronic illnesses are several times more likely to be hospitalized for influenza (PMID 36171601).

Still, the vast majority of airway infections pass without these signals. Both children and adults catch colds frequently, but symptoms usually subside within a week (PMID 21406124). Among influenza patients who received medical care, 1.2% were hospitalized (PMID 36171601).

Colds and the flu are diseases that doctors diagnose based on symptoms (PMID 16253889), acute bronchitis is also a diagnosis made through physical examination (PMID 40106287), and pneumonia is diagnosed only when chest imaging findings match as well (PMID 39283629). Symptoms alone make it difficult to distinguish whether it is pneumonia or what caused it, and that judgment is made by a doctor's physical examination and tests. The job of danger signs goes only as far as prompting you to hurry to medical care, while which room of the house that cough and fever came from and what caused them are sorted out by the examination and tests.

Where to Look Next

The cause of pneumonia in a majority of hospitalized patients ultimately remains unidentified (PMID 39283629), and there is still great uncertainty regarding what role tests that quickly find the cause, lung ultrasound, and blood markers should play in actual clinical practice (PMID 41110447).

Why the same symptoms and the same test results are read differently depending on who they come from is covered in "What a Test Actually Tells You — An Introduction to Sensitivity, Specificity, and Pretest Probability". How to read numbers like "a vaccine reduced influenza by 41%" in terms of the actual difference in the number of people can be seen in "If the Risk Was Cut in Half, How Many Fewer People Got Sick? — Relative and Absolute Risk". The spot where the food path and the airway meet in the throat is followed by "Swallowing, Speech and Language — The Crossing Where the Food Path Meets the Airway".

The defense by which airways push away dust and germs and the principle of a cough can be seen in another piece of this series, "How Do We Breathe? — An Upside-Down Tree, and the Leaves Where Air Meets Blood". Airways where wheezing repeats are taken up by "What Is Asthma? — An Airway That Tightens Too Easily, and the Inflammation That Keeps It Swollen", and airway infections suddenly worsening an old lung disease is taken over by "What Is COPD? — A Balloon That Won't Empty, and the Breath That Leaves in the First Second".

Airway infections in young children other than pneumonia — bronchiolitis, croup, and RSV prevention — are covered separately in "Bronchiolitis, RSV and Croup — In a Thin Straw, the Same Swelling Blocks More of the Way".

Source Papers

• "How and when to manage respiratory infections out of hospital." (2022) — PMID 36261157 — Lower respiratory tract infections are a heterogeneous group bundling acute bronchitis, influenza, community-acquired pneumonia, and COPD exacerbations with differing risks and management; acute bronchitis rarely requires hospitalization and usually needs no antibiotics, whereas about 40% of community-acquired pneumonia cases are hospitalized.
• "Antimicrobial treatment of ENT infections." (2023) — PMID 37730165 — Ear, nose, throat, and upper respiratory tract infections are the most common infections in children and the main driver of antibiotic prescriptions; most are caused or triggered by viruses, and even when bacteria are involved, they usually resolve on their own.
• "Prescribing practices that promote antibiotic resistance: strategies for change." (2002) — PMID 12029606 — Roughly 90% of upper respiratory tract infections are viral, yet 50% to 70% of patients who visit a clinic receive antibiotics; prescriptions with unproven benefits contribute to resistance.
• "Acute Bronchitis." (2016) — PMID 27929206 — Cough is the most common disease-related reason for outpatient visits in the United States; acute bronchitis means cough caused by acute inflammation of the trachea and large airways without evidence of pneumonia. Suspect pneumonia and obtain imaging if there is rapid breathing, rapid heart rate, shortness of breath, or abnormal lung findings. Suspect pertussis if there are paroxysmal cough spells lasting more than 2 weeks, post-tussive vomiting, or exposure. Cough lasts 2 to 3 weeks and is usually viral; antibiotics are not indicated in the absence of chronic lung disease. Antibiotics shorten the cough and illness by about half a day and carry side effects of allergy, nausea, vomiting, and C. difficile infection.
• "Diagnosis and treatment of acute bronchitis." (2010) — PMID 21121518 — Symptoms last about 3 weeks; sputum color does not reliably distinguish bacterial from viral lower respiratory tract infections; more than 90% of acute bronchitis cases are viral.
• "Acute Bronchitis: Rapid Evidence Review." (2025) — PMID 40106287 — Acute bronchitis is a self-limiting illness; antibiotics do not contribute to overall recovery, can shorten the cough by about 0.5 days, but expose patients to side effects. Delayed prescriptions and the term "chest cold" reduce prescribing.
• "Uncomplicated acute bronchitis." (2000) — PMID 11119400 — Excluding pneumonia is the primary goal in evaluating acute cough in adults without underlying conditions or hidden asthma; if vital signs (pulse, respiratory rate, oral temperature) are normal, the likelihood of pneumonia is very low; placebo-controlled trials do not support routine antibiotic treatment.
• "Influenza vaccine outcomes: a meta-analysis revealing morbidity benefits amid low infection prevention." (2025) — PMID 39778922 — Influenza A (H1N1 and H3N2) and B require annual vaccine redesign due to high antigenic drift. Conclusion: Even though H3N2 infection prevention is low, the reduction in morbidity and mortality is as high as for other subtypes.
• "Burden of influenza hospitalization among high-risk groups in the United States." (2022) — PMID 36171601 — US claims data from the 2014–2018 seasons show that among about 1.6 million patients seen for influenza, 1.2% were hospitalized. The odds ratio for hospitalization was 9.4 for those aged 65 and older compared to those aged 5–17, and 2 to 3 times higher for certain underlying conditions. Older adults with underlying conditions had a 30-day hospitalization rate 3 to 7 times higher than matched uninfected controls. Conclusion: Efforts beyond current vaccines are needed.
• "[Pneumonia and pregnancy]." (2025) — PMID 39893062 — Viral pneumonia, such as influenza, has increased severity during pregnancy; prevention relies on hygiene and vaccination.
• "Recommendations for Prevention and Control of Influenza in Children, 2025-2026: Policy Statement." (2025) — PMID 40717223 — The American Academy of Pediatrics recommends annual vaccination for all children aged 6 months and older without contraindications, and recommends antiviral treatment as early as possible regardless of illness duration for children with underlying conditions that put them at risk for hospitalization, severe illness, progression, or complications, while making it an option for other outpatients.
• "Community-Acquired Pneumonia: A Review." (2024) — PMID 39283629 — In the United States, there are about 1.4 million emergency department visits, 740,000 hospitalizations, and 41,000 deaths annually. Diagnosis equals 2 or more symptoms and signs plus matching imaging findings with no other explanation. Age 65 and older, underlying lung disease, smoking, and immunosuppression are the highest risks for sepsis, ARDS, and death. A causative pathogen is identified in only 38% of hospitalized patients, with up to 40% of those being viral. Influenza and COVID-19 testing change treatment (antivirals) and infection control.
• "Real-world effectiveness of seasonal influenza vaccination and age as effect modifier: A systematic review, meta-analysis and meta-regression of test-negative design studies." (2024) — PMID 38423813 — Across 191 test-negative design studies published between 2017 and 2022, effectiveness against confirmed influenza was 41.4%: 48.6% for those under 18, 36.7% for ages 18–64, 30.6% for ages 65 and older, and 26.8% for H3N2. Conclusion: Moderate protection, varying greatly by virus type and age.
• "Effectiveness of influenza vaccination to prevent severe disease: a systematic review and meta-analysis of test-negative design studies." (2026) — PMID 41093140 — From 165 test-negative design studies, influenza-related hospitalization was reduced by 42% (very low certainty), mortality by 36% (no certainty), pneumonia by 51% (low certainty), and intensive care unit admission by 52% (very low certainty). Effectiveness is up to 2 times higher in children and higher for H1N1 and well-matched seasons. Conclusion: Moderate reduction in severe outcomes.
• "Pneumonia." (2021) — PMID 33833230 — A common acute respiratory infection occurring in the alveoli and terminal airways, associated with short- and long-term mortality across all ages. Includes community-acquired and hospital-acquired pneumonia caused by bacteria, respiratory viruses, and fungi, with regional variations, and is common in children under 5 and older adults with underlying conditions. Diagnosis relies on clinical presentation and imaging.
• "Community-Acquired Pneumonia in Children: Rapid Evidence Review." (2021) — PMID 34913645 — Childhood pneumonia is usually viral even among hospitalized children, with RSV being most common under age 2. Findings most strongly associated with chest X-ray infiltrates are grunting, history of fever, chest wall retractions, crackles, tachypnea, and overall clinical impression.
• "Comprehensive management of pneumonia in older patients." (2025) — PMID 40021428 — Very elderly patients often present with atypical and nonspecific features leading to many false-positive diagnoses. Chest X-rays are frequently of low quality and inconclusive, so CT and chest ultrasound help confirm the diagnosis.
• "Community-acquired pneumonia." (2025) — PMID 41110447 — Community-acquired pneumonia is recognized as a disease with long-term complications such as cardiovascular events, decline in lung function, and cognitive decline. The roles of nucleic acid amplification tests, lung ultrasound, and blood biomarkers remain highly uncertain. Antibiotics are the pillar of treatment, with an emphasis on post-acute rehabilitation, cardiovascular monitoring, and vaccination.
• "Community-acquired Pneumonia and Hospital-acquired Pneumonia." (2019) — PMID 30955516 — Because the causative organism is usually unknown initially, timely empirical antibiotic administration is the pillar of pneumonia management.
• "Childhood community-acquired pneumonia." (2024) — PMID 38112800 — Conjugate vaccines have reduced the burden of the two former leading bacterial causes of childhood pneumonia over 30 years. Currently, viruses are most commonly detected. Clinical, laboratory, and imaging criteria cannot reliably distinguish bacterial from viral infections, creating an urgent need to reduce antibiotic overuse and resistance.
• "Antimicrobial prescription pattern and appropriateness for respiratory tract infection in outpatients: a systematic review and meta-analysis." (2024) — PMID 39243046 — Across 36 studies, the outpatient antibiotic prescription rate for respiratory tract infections was 66%, with an inappropriate prescription rate of 45%. The main drivers of prescriptions were acute bronchitis, pharyngitis, sinusitis, and the common cold.
• "Antibiotics for the common cold and acute purulent rhinitis." (2025) — PMID 41277585 — A Cochrane review (search updated 2013) found that for colds with symptoms lasting less than 7 days, antibiotics are no better than placebo (RR 0.83, 6 trials, 1,147 participants), with an increased risk of adverse effects (RR 1.8, 2.62 in adults, no difference in children). Conclusion: No evidence of benefit for colds and persistent purulent rhinitis, meaningful adverse effects in adult colds, and routine use not recommended.
• "Antibiotics for acute bronchitis." (2000) — PMID 11034678 — A Cochrane review of 9 trials with about 750 participants showed a cough duration reduction of about 0.58 days and an adverse effect RR of 1.48. Conclusion: Benefits are modest and of a magnitude similar to the harm from side effects; cold symptoms lasting less than a week are unlikely to benefit.
• "Appropriate Antibiotic Use for Acute Respiratory Tract Infection in Adults: Advice for High-Value Care From the American College of Physicians and the Centers for Disease Control and Prevention." (2016) — PMID 26785402 — Targeting healthy adults without chronic lung disease or immunosuppression: do not prescribe antibiotics for the common cold; do not initiate testing or antibiotics for bronchitis unless pneumonia is suspected; treat only confirmed streptococcal pharyngitis; and treat acute sinusitis only when symptoms last more than 10 days, involve high fever and purulent nasal discharge for 3 or more days, or worsen after initial improvement.
• "Antibiotic consumption and link to resistance." (2009) — PMID 19366364 — Multiple ecological studies show that increased antibiotic consumption contributes to the emergence of streptococcal resistance.
• "Antimicrobial resistance: risk associated with antibiotic overuse and initiatives to reduce the problem." (2014) — PMID 25436105 — Overuse accelerates resistance, which drives severe infections, complications, longer hospital stays, and increased mortality, with primary care respiratory tract infections being a major driver of prescriptions.
• "Systematic Review of Factors Associated with Antibiotic Prescribing for Respiratory Tract Infections." (2016) — PMID 27139474 — Across 28 studies, patient desire for antibiotics was unrelated or weakly related to prescribing, whereas physician-perceived patient desire was strongly related.
• "Immediate versus delayed versus no antibiotics for respiratory infections." (2023) — PMID 37791590 — A Cochrane review of 12 trials found that delayed prescribing reduced antibiotic use compared to immediate prescribing (30% vs. 93%), while symptom control and complications did not differ from no prescription. Conclusion: When safety is assured without immediate prescription, no prescription combined with instructions to return if needed results in the lowest usage, while delayed prescription serves as a compromise when uncertainty remains.
• "Common cold." (2011) — PMID 21406124 — A systematic review showing children get up to 5 colds a year and adults 2 to 3; symptoms generally resolve within a week, though cough may linger longer.
• "Understanding the symptoms of the common cold and influenza." (2005) — PMID 16253889 — Colds and flu are the most common infectious syndromes and are diagnosed by symptoms; cytokine action explains some of the symptoms such as fever, loss of appetite, malaise, chills, headache, and body aches.
• "Human Influenza Epidemiology." (2021) — PMID 32988982 — Most influenza virus infections are mild and self-limiting, with only a small minority being severe enough to require hospitalization; severity increases at the extremes of age and with underlying conditions, and vaccination remains the most effective tool for preventing infection.
• "Who and when to vaccinate against influenza." (2020) — PMID 32109627 — A non-systematic review noting that the benefits of vaccination in pregnant women, children aged 6 months to 5 years, older adults, and individuals with underlying medical conditions vulnerable to complications are receiving increasing attention.
• "Atypical pneumonia: Pathophysiology, diagnosis, and treatment." (2022) — PMID 34750083 — Typical bacteria have a cell wall and respond to beta-lactams, whereas most atypical pathogens lack a cell wall, requiring a different treatment approach.
• "Severity Assessment and the Immediate and Long-Term Prognosis in Community-Acquired Pneumonia." (2016) — PMID 27960212 — CURB-65 (confusion, respiratory rate, blood pressure, urea) and PSI assist clinical judgment in identifying low-risk patients; patients needing intensive care require urgent attention, and in older adults, CRB-65 no longer reliably predicts low risk, making functional status the best predictor of mortality.
• "Validity of Pneumonia Severity Assessment Scores in Africa and South Asia: A Systematic Review and Meta-Analysis." (2021) — PMID 34574976 — In adults in low- and middle-income countries, CURB-65 and CRB-65 (confusion, respiratory rate, blood pressure, age 65 and older) are valid for predicting mortality risk.