A respiratory infection is any infection caused by a virus, bacterium, or other pathogen that affects the airways or lungs. It’s one of the most common reasons people visit a doctor, ranging from mild illnesses like the common cold to serious conditions like pneumonia. Respiratory infections fall into two broad categories based on where in the airway they take hold: upper respiratory infections affect the nose, throat, and sinuses, while lower respiratory infections target the bronchial tubes and lungs.
Upper vs. Lower Respiratory Infections
Upper respiratory infections are by far the more common type. They include the common cold, sore throat (pharyngitis), sinus infections, and ear infections. These tend to be uncomfortable but manageable, and most people recover without medical treatment. The symptoms you feel, like a runny nose, scratchy throat, or congestion, reflect inflammation in the lining of your nose, sinuses, or throat.
Lower respiratory infections are generally more serious. They affect the bronchial tubes (bronchitis), the tiny air sacs in the lungs (pneumonia), or, in infants, the smallest airways (bronchiolitis). Because the lungs are directly involved, lower respiratory infections are more likely to cause difficulty breathing, chest tightness, and deep coughing. Pneumonia in particular can become dangerous, especially for older adults and young children.
What Causes Respiratory Infections
Viruses cause the majority of respiratory infections. Rhinoviruses, with over 100 different strains, are the single most common culprit, responsible for at least 25% of colds in adults. Coronaviruses account for another 10% or more. Influenza, respiratory syncytial virus (RSV), parainfluenza viruses, and adenoviruses round out the list of frequent offenders. RSV is the primary cause of bronchiolitis in infants, while parainfluenza viruses are the leading cause of croup (swelling around the voice box and windpipe) in young children.
Bacterial respiratory infections are less common but often more severe. Strep throat is caused by group A streptococcus, the most important bacterial cause of sore throat. Sinus infections and ear infections are most frequently caused by Streptococcus pneumoniae and Haemophilus influenzae. Bacterial pneumonia can develop on its own or, more commonly, as a complication following a viral illness.
How Respiratory Infections Spread
Respiratory pathogens travel between people through three main routes: respiratory droplets, smaller airborne particles, and contaminated surfaces. When someone coughs or sneezes, they release droplets of varying sizes. Larger droplets (bigger than about 5 micrometers) are heavy enough to fall to the ground within seconds. A 50-micrometer droplet, for instance, settles in roughly 20 seconds. These tend to infect people within close range.
Smaller particles tell a different story. A 5-micrometer droplet can float in still air for about 32 minutes, and a 1-micrometer particle can remain suspended for approximately 12 hours. Both influenza viruses and SARS-CoV-2 have been shown to remain viable in this aerosolized state for several hours, which is why poorly ventilated indoor spaces carry higher transmission risk. You can also pick up infections by touching a surface where droplets have landed and then touching your face, though this route is generally considered less efficient than direct inhalation.
Symptoms and How Long They Last
Most viral respiratory infections share a familiar set of symptoms: runny or stuffy nose, sore throat, cough, mild body aches, low-grade fever, and fatigue. Lower respiratory infections add shortness of breath, chest discomfort, and a deeper, more persistent cough that may produce mucus.
The timeline surprises many people. While symptoms typically peak within the first few days, they often linger. Research on children with acute respiratory infections found that roughly 25% to 32% still had symptoms two weeks after getting sick, depending on the virus involved. Around 3% to 4% experienced at least one symptom lasting beyond four weeks. Adults generally follow a similar pattern: a cough from a simple cold can persist for two to three weeks even as you feel better overall.
When a bacterial infection is present alongside a virus, illness tends to be more prolonged. Children with both viral and bacterial co-detection had fever lasting an average of 3.3 days, compared to 1.6 days for those with a virus alone. Bacterial involvement can also bring higher fevers, thicker or discolored mucus, and symptoms that worsen after an initial improvement.
Who Faces the Highest Risk
Certain groups are more vulnerable to severe illness from respiratory infections. According to the CDC, the people at highest risk include:
- Older adults: The immune system weakens with age, and older adults are more likely to have underlying health conditions that compound the effects of infection.
- Young children: Infants and toddlers have immune systems still in development, and their smaller airways make viruses that cause swelling more dangerous.
- People with chronic conditions: Heart disease, chronic lung disease, diabetes, and kidney disease all reduce the body’s ability to fight infection or affect organs that respiratory illness also targets.
- People with weakened immune systems: This includes those undergoing cancer treatment, organ transplant recipients on immune-suppressing medications, and people with primary immune deficiencies.
- Pregnant women: Pregnancy changes the immune system, heart, and lungs in ways that raise the risk of severe respiratory illness.
Environmental factors matter too. Living in congregate settings like nursing homes or dormitories, spending time in crowded and poorly ventilated spaces, and exposure to air pollution or cigarette smoke all increase susceptibility.
When Viral Infections Lead to Bacterial Complications
One of the most important things to understand about respiratory infections is how a simple viral illness can open the door to a more serious bacterial one. Bacterial pneumonia complicates an estimated 0.5% to 6% of influenza infections overall, but the rate is much higher among hospitalized and critically ill patients. During the 2009 H1N1 pandemic, bacterial co-infection was found in roughly 30% of fatal cases.
This happens through several overlapping mechanisms. A viral infection damages the protective mucus layer and the tiny hair-like structures (cilia) that sweep bacteria out of the airways. With this clearance system impaired, bacteria that normally live harmlessly in the nose and throat can migrate deeper into the lungs. The virus also strips away protective coatings on airway cells, exposing receptors that bacteria latch onto more easily.
At the same time, the immune response itself becomes compromised. Viral infections impair the function of immune cells in the lungs that normally engulf and destroy bacteria. They also reduce the chemical signals that recruit additional immune cells to the site of infection. The net result is a window of vulnerability, typically in the days following the peak of the viral illness, when bacteria can establish a foothold and cause pneumonia. This is why worsening symptoms after you’ve started to feel better, particularly a new fever, increasing shortness of breath, or a return of thick, discolored mucus, warrants prompt medical attention.
Treatment for Mild vs. Severe Cases
Most respiratory infections are viral, and viral infections don’t respond to antibiotics. For uncomplicated illnesses like the common cold and acute bronchitis, treatment is supportive: rest, fluids, and over-the-counter medications to manage fever, congestion, and cough. Acute bronchitis in otherwise healthy people is self-limited, and antibiotics do not improve outcomes.
Antibiotics become appropriate when a bacterial cause is confirmed or strongly suspected. Community-acquired pneumonia in outpatients, for example, is typically treated with antibiotics selected based on whether the person has other health conditions. For flare-ups of chronic lung disease, antibiotics are reserved for severe episodes marked by increased breathlessness, greater sputum volume, and a change in sputum color.
Oxygen levels serve as an important indicator of severity. An oxygen saturation reading at or below 94% is a signal that in-person medical evaluation is needed. Saturations below 95% are an independent predictor of confirmed pneumonia on a chest X-ray. Inexpensive pulse oximeters can give you a quick reading at home, though they’re less accurate in people with darker skin tones or poor circulation.
Prevention and Vaccination
Vaccines are the most effective tool for preventing severe respiratory infections. Flu vaccines are recommended for everyone aged 6 months and older. For the 2025-2026 flu season, the CDC recommends single-dose, preservative-free formulations for children, pregnant women, and adults. A nasal spray option (FluMist) is available for people ages 2 through 49, and as of 2024, it can be self-administered at home by adults or given by a caregiver for children.
Beyond the flu shot, vaccines targeting RSV and pneumococcal disease are available for the groups at highest risk. RSV vaccines and preventive antibody treatments protect newborns, young infants, and older adults. Pneumococcal vaccines help prevent bacterial pneumonia and are particularly important for older adults and people with chronic health conditions.
Everyday prevention habits still matter. Good ventilation in indoor spaces reduces the concentration of airborne particles. Regular handwashing limits surface-based transmission. Staying home when you’re symptomatic, especially during the first few days when viral shedding peaks, protects the people around you.