A respiratory infection is any infection caused by a virus, bacterium, or fungus that affects the airways you use to breathe. These infections range from mild (like the common cold) to life-threatening (like pneumonia), and they are among the leading causes of illness and death worldwide. Lower respiratory infections alone were responsible for an estimated 2.5 million deaths globally in 2023, with the heaviest toll falling on children under five and adults over 70.
Upper vs. Lower Respiratory Infections
The respiratory tract is divided into two zones, and infections are classified by which zone they affect. Your upper respiratory tract includes your nose, mouth, nasal cavity, sinuses, and larynx (voice box). Your lower respiratory tract starts at the trachea (windpipe) and extends into the bronchi and lungs.
Upper respiratory infections are the more common and generally milder category. They include the common cold, sinus infections, tonsillitis, and laryngitis. Symptoms tend to stay above the chest: runny or stuffy nose, sore throat, sneezing, mild cough, and sometimes a low fever. Most resolve on their own within a week or two.
Lower respiratory infections affect the airways deeper in your chest and lungs. Bronchitis, bronchiolitis, and pneumonia all fall into this group. These infections tend to last longer and can be more serious, especially in vulnerable people. Symptoms often include a persistent or worsening cough, chest tightness, shortness of breath, higher fevers, and fatigue that lingers. Pneumonia, a lung infection, is the most dangerous of the common types.
What Causes Respiratory Infections
Viruses cause the vast majority of respiratory infections. Rhinoviruses and coronaviruses are the most frequent culprits behind the common cold. Influenza viruses dominate during flu season. RSV (respiratory syncytial virus) is a leading cause of bronchiolitis in infants and can cause serious pneumonia in older adults. Parainfluenza viruses are the primary trigger for croup in young children. Adenoviruses, enteroviruses, and human metapneumoviruses round out the list of common viral causes.
Bacteria play a smaller but significant role. Streptococcus pneumoniae is the single largest bacterial killer in lower respiratory infections, accounting for roughly 25% of all deaths from these infections worldwide, about 634,000 in 2023. Staphylococcus aureus and Klebsiella pneumoniae are the next most deadly. On the upper tract side, group A strep is the classic cause of strep throat, and bacteria can also cause sinus infections and ear infections.
How Respiratory Infections Spread
Most respiratory infections spread through droplets expelled when an infected person coughs, sneezes, or talks. Larger droplets (bigger than 5 to 10 micrometers) travel short distances and settle quickly, which is why being within about one meter of a sick person raises your risk significantly. Smaller particles, under 5 micrometers, can linger in the air longer and travel farther, a process sometimes called airborne or aerosol transmission.
You can also pick up infections by touching contaminated surfaces and then touching your mouth, nose, or eyes. This is why handwashing remains one of the simplest and most effective defenses. The virus doesn’t need to be alive in large quantities on a surface to pose a risk, but direct person-to-person droplet exposure is the primary route for most respiratory pathogens.
Who Is Most at Risk
Certain groups face a much higher chance of developing severe illness from respiratory infections. Most deaths from respiratory viruses occur in people older than 65, with risk climbing sharply with advancing age. Aging weakens the immune system and makes it harder to fight off infections, and older adults are more likely to have other health conditions that compound the danger.
Young children, particularly infants, are also highly vulnerable. Their immune systems are still developing, and their smaller airways make infections that cause swelling more dangerous. Children under five had a mortality rate from lower respiratory infections of about 95 per 100,000 in 2023, though this figure has dropped by roughly a third since 2010.
Other high-risk groups include people with chronic lung disease, heart disease, diabetes, or kidney disease. People with weakened immune systems from cancer treatment, organ transplants, or immunosuppressive medications face particular danger because their bodies have a harder time mounting a defense. Pregnant women are at increased risk because pregnancy changes immune function and puts extra strain on the heart and lungs. People with certain disabilities may also face elevated risk due to underlying conditions or living situations.
Symptoms to Pay Attention To
Most upper respiratory infections feel like a nuisance: a stuffy nose, scratchy throat, mild cough, and maybe a low-grade fever. These symptoms typically peak within two to three days and gradually improve over a week.
Lower respiratory infections feel different. A cough that produces colored mucus, chest pain or pressure when breathing, shortness of breath, and fevers above 101°F (38.3°C) all suggest the infection has moved into the lungs or lower airways. Rapid breathing, wheezing, and feeling winded during normal activities are signs the infection may be worsening. In infants and very young children, watch for fast breathing, difficulty feeding, unusual fussiness, or a bluish tint around the lips.
How Respiratory Infections Are Diagnosed
For common colds and mild infections, a diagnosis usually comes from your symptoms alone. When a specific virus needs to be identified, or when the infection is more serious, testing becomes useful.
Rapid antigen tests are the fastest option, returning results in about 15 to 30 minutes. At-home tests for COVID-19 and flu fall into this category. They’re convenient but not as sensitive as lab-based tests, meaning they can miss infections, especially when viruses are circulating heavily. Multiplex self-tests that check for both COVID-19 and flu in a single swab are now available, and molecular versions of these are more accurate than antigen-based ones.
Molecular tests, including PCR, detect the actual genetic material of the virus and are more reliable. They typically require a sample collected by a healthcare provider and sent to a lab, though some point-of-care versions deliver faster results. For suspected bacterial infections like strep throat, a rapid strep test or throat culture can confirm the diagnosis. For pneumonia, a chest X-ray is often used to check for fluid or inflammation in the lungs.
Treatment: Viral vs. Bacterial
This distinction matters because antibiotics only work against bacteria. They are ineffective against the common cold, flu, COVID-19, and laryngitis. Taking antibiotics for a viral infection won’t speed recovery and contributes to antibiotic resistance, a growing public health problem.
For viral upper respiratory infections, treatment is supportive: over-the-counter pain relievers, antihistamine and decongestant combinations, rest, and fluids. That short list is actually all that has established effectiveness for cold symptoms in adults. Antiviral medications exist for specific viruses like influenza and COVID-19, and they work best when started early in the illness.
Bacterial infections do warrant antibiotics. Strep throat is typically treated with a 10-day course of penicillin-type antibiotics, along with pain and fever reducers for comfort. Bacterial sinus infections in adults are treated with a five to seven day antibiotic course. For ear infections in children, the approach depends on age and severity. Children two and older with mild symptoms may be monitored for a few days before starting antibiotics, since many ear infections clear on their own.
Potential Complications
Most respiratory infections resolve without lasting effects, but complications can develop, particularly in high-risk individuals. An upper respiratory infection can progress to a lower one if the pathogen spreads deeper into the airways. A cold can lead to a sinus infection, ear infection, or trigger an asthma flare. Pneumonia can develop as a secondary bacterial infection on top of an initial viral illness.
The most dangerous complication is sepsis, which happens when an infection triggers an extreme immune response that damages your own tissues and organs. Lung infections are among the most common starting points for sepsis. Without prompt treatment, sepsis can progress to organ failure and death. Signs of sepsis include confusion, extreme pain, rapid heart rate, fever or very low body temperature, and feeling sicker than you’d expect.
Prevention and Vaccination
Basic hygiene measures remain your first line of defense: regular handwashing, avoiding close contact with sick individuals, and covering coughs and sneezes. Staying home when you’re symptomatic helps break the chain of transmission.
Vaccines offer more targeted protection against the most dangerous respiratory pathogens. The annual flu vaccine reduces the risk of hospitalization by roughly 50 to 78% in children and 38 to 57% in adults, depending on the season and the match between the vaccine and circulating strains. These numbers vary year to year, but even in a season with lower effectiveness, vaccination reduces severity and the chance of complications. Pneumococcal vaccines protect against the bacterium responsible for the largest share of lower respiratory infection deaths globally. RSV vaccines are now available for older adults and, through maternal vaccination, for newborns.
No single measure eliminates the risk entirely, but the combination of vaccination, hand hygiene, and sensible avoidance of exposure during peak illness seasons meaningfully lowers your chances of both catching and spreading respiratory infections.