The four main types of respiratory infections are the common cold, influenza (flu), bronchitis, and pneumonia. These fall into two broad categories based on where they strike: upper respiratory infections affect everything from your vocal cords upward (nose, sinuses, throat), while lower respiratory infections target the airways and air sacs below the vocal cords. Lower respiratory infections remain the world’s leading infectious cause of death, responsible for roughly 2.5 million deaths globally in 2023, with children under 5 and adults over 70 carrying the heaviest burden.
The Common Cold
The common cold is the most frequent respiratory infection people experience. Rhinoviruses cause the majority of colds, though dozens of other viruses can trigger one too. Symptoms include a runny or stuffy nose, sneezing, sore throat, cough, headache, mild body aches, and sometimes a low fever. Most colds resolve in under 7 days, though symptoms can linger for up to 2 weeks.
Colds have one of the shortest incubation periods of any respiratory infection, just 12 hours to 3 days from exposure to first symptoms. You’re contagious before you even realize you’re sick, since the infectious window can overlap with the tail end of incubation. This is a big reason colds spread so efficiently through households and workplaces. Treatment is purely about comfort: saline nasal drops to loosen thick mucus, warm moist air, staying hydrated, and rest. Antibiotics do nothing for a cold because it’s viral.
Influenza (Flu)
Influenza hits harder and faster than a cold. There are four types of influenza viruses (A, B, C, and D), but only A and B cause the seasonal epidemics that sweep through populations each year. Influenza A is further divided into subtypes based on surface proteins, with A(H1N1) and A(H3N2) currently circulating in humans. Influenza C causes mild illness and rarely makes headlines, while influenza D primarily affects cattle and doesn’t infect people.
Globally, about a billion cases of seasonal influenza occur each year, including 3 to 5 million cases of severe illness. The incubation period runs 1 to 4 days, and you’re most contagious during the first 3 days of symptoms, though you can spread the virus a day before symptoms appear. What separates flu from a cold is the intensity: sudden onset of high fever, significant body aches, fatigue that keeps you in bed, and a dry cough. In severe cases, influenza can progress to pneumonia or sepsis, which is why it’s particularly dangerous for older adults, young children, and people with weakened immune systems.
Bronchitis
Bronchitis is an infection of the bronchial tubes, the airways that connect your windpipe to your lungs. It can be caused by viruses or bacteria, though viral bronchitis is far more common. The hallmark symptom is a persistent cough that often produces yellow-green mucus as the infection progresses. Other symptoms overlap with upper respiratory infections: sore throat, runny nose, mild fever, body aches, headache, wheezing, and fatigue.
Acute bronchitis typically develops after a cold or flu, when the initial infection spreads deeper into the airways. The cough can stick around for weeks even after other symptoms clear, which often alarms people, but it usually reflects lingering irritation rather than worsening illness. Because most cases are viral, antibiotics aren’t helpful. Warm, moist air and staying well hydrated can ease the discomfort while your body clears the infection.
Pneumonia
Pneumonia is the most serious of the four types. While bronchitis affects the airways leading to the lungs, pneumonia infects the alveoli, the tiny air sacs deep in the lungs responsible for transferring oxygen into your bloodstream. This is why pneumonia causes more severe, body-wide symptoms: your organs simply aren’t getting enough oxygen. Pneumonia can be bacterial, viral, or fungal, and it sometimes develops as a secondary infection after the flu or another respiratory illness.
Symptoms include rapid breathing or shortness of breath, high fever (potentially reaching 105°F), chills, sweating, chest or abdominal pain when coughing, confusion, and loss of appetite. These overlap with bronchitis symptoms but tend to be significantly more intense and last longer. Bacterial pneumonia typically requires antibiotics, while viral pneumonia is managed with supportive care. Pneumonia is the primary reason lower respiratory infections rank as such a major global killer, especially in very young children and older adults.
RSV: A Common Cause Across Age Groups
Respiratory syncytial virus, or RSV, deserves mention because it can cause several of the infections listed above, from mild cold-like illness to bronchitis and pneumonia. Its incubation period is 4 to 6 days. In infants and young children, RSV often starts with a runny nose and decreased appetite before progressing to cough, sneezing, fever, and wheezing. Very young infants may show irritability, decreased activity, or pauses in breathing. In adults over 50, RSV typically looks like an upper respiratory infection with runny nose, sore throat, cough, headache, and fatigue.
Vaccines are now available for high-risk groups. A single dose is recommended for all adults 75 and older, and for adults 50 to 74 who face increased risk of severe RSV. For infants, protection comes either through a maternal vaccine given during weeks 32 to 36 of pregnancy or through a preventive antibody given to babies after birth.
How to Tell Viral From Bacterial
One of the most practical distinctions with respiratory infections is whether they’re caused by a virus or bacteria, because this determines whether antibiotics will help. Most respiratory infections are viral, meaning antibiotics are useless and the illness needs to run its course with supportive care. Bacterial infections, on the other hand, require targeted treatment.
Doctors use a few clues to differentiate. Bacterial infections tend to produce higher levels of inflammation markers in blood tests. In one study, patients with bacterial respiratory infections had inflammation levels roughly four times higher than those with viral infections. But in everyday practice, doctors often rely on symptom patterns: a fever that spikes after initially improving, symptoms that worsen rather than gradually getting better after a week, or thick discolored mucus that persists beyond 10 days can all point toward a bacterial cause. Prescribing guidelines emphasize confirming a bacterial diagnosis before reaching for antibiotics, selecting the most effective option, and treating for the shortest effective duration.
When Infections Overlap or Progress
These four types aren’t always neatly separated. A cold can trigger bronchitis. The flu can lead to pneumonia. RSV can start as a runny nose and end up deep in the lungs. This cascading pattern is especially common in people with weaker immune defenses, whether from age, chronic illness, or other factors.
The key warning signs that a mild respiratory infection is becoming something more serious include shortness of breath, chest pain, a fever that returns after seeming to improve, confusion, or symptoms that keep getting worse after 7 to 10 days instead of trending toward recovery. These shifts often signal that the infection has moved deeper into the respiratory tract or that a secondary bacterial infection has set in on top of the original viral one.