You get a respiratory infection by breathing in virus-laden particles, touching a contaminated surface and then touching your face, or being in close contact with someone who is infected, even if they show no symptoms. The most common route is inhaling droplets or tiny airborne particles released when an infected person coughs, sneezes, talks, or simply breathes.
Droplets, Aerosols, and Close Contact
Respiratory viruses travel between people in two main forms. Larger respiratory droplets (bigger than 5 to 10 micrometers) are heavy enough that they fall to the ground relatively quickly, usually within about one meter of the person who produced them. This is why standing close to someone who is coughing or sneezing is one of the fastest ways to pick up an infection. These droplets can land directly on your mouth, nose, or eyes.
Smaller particles, called droplet nuclei (under 5 micrometers), behave differently. They’re light enough to hang in the air for extended periods and travel well beyond one meter. In poorly ventilated rooms, these tiny particles can accumulate over time, which is why crowded indoor spaces with stale air are particularly risky during cold and flu season. You don’t need to be standing next to a sick person to inhale them.
Surface Contact and Hand-to-Face Transfer
Viruses that land on surfaces can survive long enough for someone else to pick them up. On hard, non-porous surfaces like plastic and stainless steel, many respiratory viruses remain viable for hours, with some lasting up to several days at room temperature. Porous surfaces like fabric and paper tend to deactivate viruses faster, sometimes within minutes, though survival times of one to five hours are common.
The chain of infection works like this: an infected person coughs into their hand, touches a doorknob or countertop, and you touch the same spot minutes or hours later. The virus doesn’t infect you through your skin. It gets in when you touch your nose, rub your eyes, or eat without washing your hands. Most people touch their face dozens of times per hour without realizing it, which makes this route surprisingly effective.
You Can Catch It From Someone Who Looks Healthy
One of the trickiest aspects of respiratory infections is that people can spread them before they feel sick, or without ever developing symptoms at all. Research on ambulatory populations has found that, depending on how symptoms are defined, anywhere from 65% to 97% of detected respiratory infections were classified as asymptomatic. Among children specifically, 25% to 64% of rhinovirus infections (the most common cause of colds) produce no noticeable symptoms.
Older adults tend to show symptoms more often when infected. Studies in elderly populations found that 80% to 90% of those infected with RSV, influenza, or similar viruses were symptomatic. Still, that leaves a meaningful percentage shedding virus while feeling fine. The practical takeaway: you can’t reliably tell who around you is contagious just by looking for coughs and sniffles.
What Happens Once the Virus Reaches Your Airways
Once inhaled or transferred to your nose or mouth, the virus needs to latch onto cells lining your respiratory tract. Different viruses use different strategies to do this. Influenza viruses, for example, are pulled inside cells through a process triggered by the acidic environment of tiny cellular compartments. RSV uses surface proteins to grab onto receptors on your airway cells, then essentially tricks the cell into pulling the virus inside through a process resembling the cell’s normal intake of nutrients.
After entering a cell, the virus hijacks its machinery to produce copies of itself. Those copies burst out and infect neighboring cells, and the cycle repeats. Your immune system detects the invasion and mounts an inflammatory response, which is what produces most of the symptoms you feel: sore throat, congestion, cough, fever, and fatigue. The infection itself doesn’t always cause direct damage. Much of your misery comes from your own body fighting back.
How Long Before Symptoms Appear
The gap between exposure and the first signs of illness varies depending on the virus:
- Common cold (rhinovirus): 12 hours to 3 days
- Influenza: 1 to 4 days
- COVID-19: 2 to 14 days, with an average of 3 to 4 days for current variants
- RSV: 4 to 6 days
- Adenovirus: typically 5 to 6 days, ranging from 2 to 14
During part or all of this incubation period, you may already be contagious. This is especially true for influenza, where viral shedding often begins a full day before symptoms start, and COVID-19, where presymptomatic transmission has been well documented.
Who Is Most Vulnerable
Everyone is susceptible to respiratory infections, but certain groups face a higher risk of both catching them and developing serious complications. According to the CDC, the people at greatest risk include older adults, young children, pregnant women, people with weakened immune systems, and those with chronic conditions like lung disease, heart disease, diabetes, or kidney disease.
Age plays a role at both ends of the spectrum. In older adults, the immune system weakens over time, and most deaths from respiratory viruses occur in people over 65, with risk increasing sharply with advancing age. In young children, especially infants, the immune system is still developing, and their smaller airways make viruses that cause swelling more dangerous.
Pregnancy shifts immune function, heart output, and lung capacity in ways that make respiratory infections harder to fight off. People on immunosuppressive medications, such as those being treated for cancer or managing an organ transplant, have lower defenses and may not build lasting protection even after vaccination or a prior infection. People with disabilities may face compounding risks from underlying conditions, congregate living settings, or barriers to healthcare access.
Practical Ways to Lower Your Risk
Preventing respiratory infections comes down to reducing the number of virus particles that reach your airways and keeping your immune system prepared to deal with the ones that do. The CDC’s core prevention strategies center on a few key habits: staying current on recommended vaccines, washing your hands regularly, improving indoor air quality, and staying home when you’re sick.
Hand hygiene works because it breaks the surface-to-face chain of transmission. Soap and water for 20 seconds is effective; alcohol-based hand sanitizer works when a sink isn’t available. Cleaning commonly touched surfaces, like phones, keyboards, and light switches, reduces the viral load in your environment.
Air quality matters more than most people realize. Opening windows, using air purifiers with HEPA filters, or simply spending more time outdoors all reduce the concentration of airborne particles in shared spaces. In settings where respiratory illness is circulating heavily, masks and physical distancing add extra layers of protection. Testing when you have symptoms helps you make informed decisions about isolating and seeking treatment, particularly if you fall into a higher-risk group where early treatment can reduce the chance of severe illness.