Upper respiratory infections are caused by viruses in the vast majority of cases. More than 200 different respiratory viruses can trigger an infection in the nose, throat, or voice box, with rhinoviruses being the single most common culprit. Adults typically catch three to six of these infections per year, while young children can come down with as many as eight, or up to 14 if they attend daycare.
Where the Infection Takes Hold
An upper respiratory infection, or URI, affects the passages above your lungs: the nostrils, nasal cavity, mouth, throat (pharynx), and voice box (larynx). When a virus lands on the lining of any of these structures, it begins replicating in the cells there, triggering inflammation. That inflammation is what produces the familiar symptoms: congestion, sore throat, runny nose, cough, and sometimes a low fever. The infection stays in the upper airway, which is what distinguishes a URI from lower respiratory infections like bronchitis or pneumonia.
The Viruses Behind Most Infections
Rhinoviruses are the most frequent cause of URIs in the United States and the primary driver of what people call “the common cold.” Beyond rhinoviruses, a long list of viral families can produce nearly identical symptoms: common human coronaviruses (not just the one behind COVID-19), parainfluenza viruses, adenoviruses, enteroviruses, respiratory syncytial virus (RSV), and human metapneumovirus. Because so many different viruses cause URIs, you can catch several per year without ever being infected by the same virus twice.
These viruses also have different incubation periods, which is the gap between exposure and the first sign of symptoms. A common cold can show up in as little as 12 hours, though it usually takes one to three days. Influenza has a one-to-four-day window. Adenoviruses are slower, averaging five to six days but occasionally taking up to two weeks. RSV falls in the four-to-six-day range. Knowing these timelines can help you trace back to where you likely picked up an infection.
When Bacteria Are the Cause
A small fraction of URIs are bacterial rather than viral. Group A Streptococcus is the bacterium responsible for strep throat, which is one of the most common bacterial URIs and is especially prevalent in school-age children. Bordetella pertussis, the bacterium behind whooping cough, accounts for roughly 95% of pertussis cases, with a closely related species causing the rest.
Bacteria can also move in after a virus has already weakened the tissue lining your airways. This is called a secondary bacterial infection, and there are a few signs it may be happening: symptoms that persist well beyond the 10 to 14 days a virus typically lasts, a fever that spikes higher than expected, or a fever that initially improves and then worsens several days into the illness. These patterns suggest the original viral infection has created an opening for bacteria to take hold.
How URIs Spread From Person to Person
Respiratory viruses travel between people through two main routes. The first is respiratory droplets, which are tiny particles larger than 5 to 10 micrometers that get launched into the air when someone coughs, sneezes, talks, or even breathes. These heavier droplets tend to fall to the ground within a few feet, which is why close contact carries the highest risk. Smaller particles under 5 micrometers, sometimes called aerosols, can linger in the air longer and travel further, particularly in poorly ventilated indoor spaces.
The second route is contact transmission. Viruses land on surfaces like doorknobs, phones, and countertops and can remain viable for hours to days depending on the surface material, temperature, and humidity. You pick up the virus on your hands and then touch your nose, mouth, or eyes. This is why hand hygiene is so effective at breaking the chain. A meta-analysis of 12 studies found that combining face masks with thorough handwashing significantly reduced the risk of respiratory infection transmission, with mask use alone cutting the odds by about 34%. Compliance matters, though. Occasional or inconsistent use offered far less protection than consistent, early adoption of both habits together.
Environmental Conditions That Raise Your Risk
Cold, dry weather is consistently linked to higher rates of respiratory infection. A 2024 scoping review in Frontiers in Environmental Health analyzed dozens of studies and found that case rates were generally higher at lower temperatures and lower humidity levels. Cold air dries out the mucous membranes in your nose and throat, reducing their ability to trap and clear viruses before they can infect cells. Low humidity also helps viral particles stay airborne longer.
Air pollution plays a role too. The same review found that 22 studies reported higher case numbers in regions or time periods with elevated air pollution. Pollutants irritate and inflame the airways, which can weaken local immune defenses and make it easier for a virus to establish an infection. Living near heavy traffic, industrial areas, or in cities with poor air quality can increase your baseline susceptibility to URIs beyond what weather alone would predict.
Who Gets Sicker and Why
While most URIs are mild and resolve on their own, certain groups face a higher risk of the infection becoming severe or progressing into something worse. Young children, particularly infants, have immune systems that are still developing and smaller airways that are more easily obstructed by inflammation. Older adults face the opposite end of the same problem: immune function naturally weakens with age, and they are more likely to have underlying conditions that compound the risk.
Chronic health conditions change the equation significantly. Chronic lung disease, heart disease, diabetes, and chronic kidney disease all increase the chance that a routine URI escalates. People with weakened immune systems, whether from cancer treatment, organ transplant medications, or a primary immune deficiency, have lower defenses against infection from the start. Pregnancy also raises the stakes because of changes to the immune system, heart, and lungs that occur naturally during gestation. People with certain disabilities may face compounding factors, including underlying medical conditions, living in group settings where viruses circulate easily, and barriers related to healthcare access.
Lifestyle Factors That Lower Your Defenses
Beyond chronic conditions, everyday habits influence how often you catch URIs and how well your body fights them off. Sleep deprivation suppresses the immune cells your body relies on to detect and destroy viruses in the early hours of an infection. Consistently getting fewer than six or seven hours of sleep leaves you measurably more vulnerable. Smoking and regular exposure to secondhand smoke damage the cilia, the tiny hair-like structures lining your airways that sweep mucus and trapped pathogens out of the respiratory tract. With those cilia impaired, viruses have an easier path to infecting cells.
Crowded environments, whether schools, offices, public transit, or daycare centers, concentrate the very conditions viruses need to spread: close contact, shared surfaces, and recirculated indoor air. The daycare statistic is striking: children in group childcare settings can contract up to 14 URIs a year, nearly double the upper range for children cared for at home. For adults, similar dynamics play out in open-plan offices and during peak commuting hours, especially in the colder months when windows stay shut and ventilation drops.