You get an upper respiratory infection (URI) by breathing in virus-laden particles from an infected person or by touching a contaminated surface and then touching your eyes, nose, or mouth. The vast majority of URIs are caused by viruses, not bacteria, and the common cold alone accounts for millions of infections each year. Understanding exactly how these germs spread, and what makes some people more vulnerable, can help you reduce your risk.
How the Virus Reaches You
Upper respiratory infections spread through three main routes, and most people catch them through a combination of all three.
Respiratory droplets are the primary route. When someone with a cold or flu coughs, sneezes, or even talks, they release droplets larger than 5 to 10 micrometers in diameter. These droplets travel short distances and can land on your mouth, nose, or eyes if you’re within about one meter (roughly three feet) of the infected person. This is why close, face-to-face contact with a sick person is the single biggest risk factor for catching a URI.
Smaller airborne particles (under 5 micrometers) can linger in the air longer and travel farther than one meter. This is especially relevant in poorly ventilated indoor spaces like classrooms, offices, or public transit, where tiny viral particles accumulate over time.
Contaminated surfaces are the third route. If an infected person coughs into their hand and then touches a doorknob, phone, or countertop, the virus can survive there for several hours to days depending on the surface type. Some viruses last longer on hard surfaces like stainless steel and plastic, while others persist better on fabrics. You get infected when you touch that surface and then touch your face, which most people do dozens of times per hour without realizing it.
Which Viruses Cause URIs
Common cold viruses, particularly rhinoviruses, are responsible for the largest share of upper respiratory infections. But they’re far from the only culprits. Influenza A and B, RSV, COVID-19, adenoviruses, and even herpes simplex and varicella-zoster viruses can all infect the upper airways. Each virus behaves a little differently in terms of how fast symptoms appear, how long you’re contagious, and how severe the illness gets.
Because so many different viruses cause URIs, you can get several infections in a single year. Immunity to one rhinovirus strain doesn’t protect you against the dozens of others circulating at any given time, which is why “catching a cold” is such a common, recurring experience.
How Quickly Symptoms Appear
The gap between exposure and your first symptoms varies by virus. For the common cold, it’s remarkably fast: as little as 12 hours and usually no more than three days. The flu takes one to four days. RSV typically needs four to six days to cause symptoms. COVID-19 has a wider window, anywhere from two to 14 days, though the Omicron variants average three to four days. Adenoviruses usually take five to six days but can incubate for up to two weeks.
During this incubation period, you may already be contagious even though you feel perfectly fine. This is one of the reasons URIs spread so efficiently: people unknowingly pass the virus along before they have any reason to stay home or take precautions.
Why Some People Get Sick More Often
Two people can be exposed to the exact same virus and have completely different outcomes. Research from Carnegie Mellon University’s stress and immunity lab has shown that psychological stress is one of the strongest predictors of whether an exposed person actually develops a cold. In controlled studies where volunteers were deliberately exposed to cold viruses, those reporting higher stress levels developed clinical illness at significantly higher rates, and this held true across all five virus types tested. The relationship was dose-dependent: more stress meant more illness.
Interestingly, the link between stress and getting sick wasn’t simply explained by the fact that stressed people smoke more, sleep worse, or eat poorly, though those habits do suppress immune function on their own. Even after accounting for those behaviors, stress itself independently increased vulnerability. People experiencing more negative emotions before exposure also reported worse symptoms and produced more nasal mucus during illness.
Sleep deprivation, smoking, a poor diet, and lack of exercise all weaken the body’s first-line defenses in the nose and throat. Young children, older adults, and anyone with a chronic condition affecting the immune system are also at higher risk. Spending time in crowded indoor spaces, especially during fall and winter when people are indoors with windows closed, dramatically increases your exposure.
Handwashing, Sanitizer, and Prevention
Hand hygiene is widely recommended to prevent URIs, but the evidence on how to do it best is more nuanced than you might expect. A systematic review published in BMJ Open compared soap-and-water handwashing to alcohol-based hand sanitizer across multiple randomized trials. Hand sanitizer reduced respiratory infection events by about 15% compared to no intervention, a statistically significant benefit. Soap and water, surprisingly, did not show a significant reduction in the pooled analysis, though individual trials varied.
In one trial at Swedish childcare centers, adding an alcohol-based sanitizer after regular handwashing reduced absenteeism by 12%. A Spanish study of 24 childcare centers found children in the sanitizer group had a 13% lower risk of respiratory infection than those using soap and water alone. However, a trial in Kenyan primary schools found no meaningful difference between the two methods.
The practical takeaway: hand sanitizer with at least 60% alcohol is a convenient and effective option, especially when you’re out in public and can’t get to a sink. Soap and water is still valuable for removing visible dirt and certain pathogens that alcohol doesn’t kill well, but for preventing respiratory viruses specifically, keeping a bottle of hand sanitizer accessible may be the more reliable habit.
Other Ways to Lower Your Risk
Beyond hand hygiene, a few strategies meaningfully reduce your chances of picking up a URI. Avoiding touching your face is simple advice but remarkably hard to follow. Keeping your distance from people who are visibly sick, improving ventilation in indoor spaces (even cracking a window helps), and staying up to date on flu and COVID vaccines all reduce either your exposure or the severity of illness if you do get infected.
Managing stress and protecting your sleep may matter more than most people realize. Given the strong, consistent link between psychological stress and cold susceptibility, habits like regular physical activity, adequate sleep (seven or more hours for most adults), and whatever stress-management techniques work for you aren’t just general wellness advice. They directly affect whether your immune system can fight off the next virus you encounter.