People get the flu by inhaling virus-laden particles that an infected person expels when they cough, sneeze, talk, or even breathe. Less commonly, you can pick up the virus by touching a contaminated surface and then touching your nose, mouth, or eyes. What makes influenza so effective at spreading is that it travels in particles of varying sizes, some of which can hang in the air for over an hour, and infected people start shedding the virus a full day before they feel sick.
Respiratory Droplets and Airborne Particles
When someone with the flu coughs or sneezes, they release a burst of particles in a wide range of sizes. The larger droplets, those bigger than about 20 micrometers across, fall to the ground within seconds. These are the ones you’d catch by standing close to a sick person, typically within about six feet. But a large number of expelled particles are smaller than 10 micrometers, and these behave very differently.
Small particles evaporate rapidly after leaving the body, shrinking to roughly half their original size. At that point they become what scientists call droplet nuclei: tiny enough to stay suspended in the air for extended periods. A 5-micrometer particle takes over an hour to fall just 10 feet. Particles smaller than 3 micrometers essentially don’t settle at all and can drift across a room or through ventilation systems. These fine aerosols penetrate deep into the lungs, bypassing the natural traps in your nose and throat, which may explain why aerosol exposure can cause infection with a remarkably small amount of virus. Human challenge studies have found that fewer than five infectious units delivered as an aerosol can be enough to establish an infection in the nose.
How the Virus Gets Into Your Cells
Once influenza particles land on the lining of your respiratory tract, the virus uses a protein on its surface to latch onto sugar molecules (sialic acid) that coat your airway cells. Think of it as a lock-and-key system: the viral protein is shaped to recognize a specific sugar arrangement found on human cells. This binding is the first step. Once attached, the virus tricks the cell into pulling it inside, where it hijacks the cell’s machinery to make copies of itself. Within hours, newly produced viruses burst out and infect neighboring cells, and the cycle accelerates.
This lock-and-key specificity is also why bird flu strains don’t easily infect humans. Avian influenza viruses are shaped to bind a slightly different version of the same sugar. Sometimes a single mutation in the virus is enough to flip that preference, which is one reason public health officials closely monitor bird flu outbreaks.
Touching Contaminated Surfaces
Flu viruses don’t only travel through the air. When virus-containing droplets land on surfaces, they can remain infectious for surprisingly long periods depending on the material. On stainless steel, viable flu virus has been detected for up to two weeks. On fabric like cotton, the virus survives for about a week, though its numbers drop much faster: cotton reduces 99% of viable virus in roughly 18 hours, while stainless steel takes about 175 hours to reach the same reduction. Hard, nonporous surfaces are the bigger concern for indirect transmission.
You can’t get the flu through your skin. The risk comes when you touch a doorknob, phone, or countertop carrying the virus and then touch your face. Your eyes, nose, and mouth all provide entry points to the mucous membranes where influenza can take hold.
When Infected People Are Contagious
One of the trickiest aspects of flu transmission is the timing. Infected people begin shedding virus about one day before any symptoms appear. That means someone who feels perfectly fine can already be spreading the flu at work, school, or home. Once symptoms begin, a person is most contagious during the first three days of illness. Viral shedding typically continues for five to seven days after symptoms start, though young children and people with weakened immune systems can remain contagious longer.
Not everyone who catches the flu knows they have it. Roughly 36% of influenza infections produce no symptoms at all. These asymptomatic carriers are less infectious than people who are visibly sick, estimated to be about 57% as contagious, but they still account for an estimated 26% of household transmission. That’s a significant share of spread coming from people who never realize they’re infected.
Why Flu Spreads More in Winter
Influenza follows a strong seasonal pattern, peaking in colder months in temperate climates. The key factor turns out to be absolute humidity, the actual amount of water vapor in the air, rather than temperature alone. Absolute humidity explains about 50% of the variation in transmission efficiency and 90% of the variation in how long the virus survives outside a body.
In cold, dry air, two things work in the virus’s favor. First, expelled droplets evaporate faster, producing smaller aerosol particles that float longer and travel farther. Second, the virus itself stays viable longer when the air is dry. In warm, humid conditions, droplets stay large and fall quickly, and the virus degrades faster. This is why summer flu outbreaks are rare in places with distinct seasons, and why heated indoor air in winter, which is often very dry, creates ideal conditions for the virus to spread between people in close quarters.
Who Is More Vulnerable to Infection
Everyone with a functioning respiratory tract can catch the flu, but some people are more susceptible to infection taking hold. People with asthma have airway cells that mount a weaker initial immune response to the virus. Their immune defenses, particularly the signaling molecules that coordinate the body’s first-line antiviral attack, are blunted compared to healthy airways. The corticosteroid medications commonly used to manage asthma may further suppress local immune function, compounding the problem.
Young children are more vulnerable partly because their immune systems haven’t encountered many flu strains yet, leaving them without the partial immunity that repeated exposures build over a lifetime. Older adults face the opposite issue: their immune systems have seen many strains but respond more slowly and less forcefully to new ones. Pregnant women, people with diabetes, and those on immunosuppressive medications also face higher risk of infection progressing to serious illness.
How to Reduce Your Risk
Hand washing is one of the most effective ways to prevent indirect transmission, and it works better than you might expect. Plain water alone inactivates influenza virus on hands within 30 seconds, even when the virus is still wrapped in wet mucus. Alcohol-based hand sanitizers, by contrast, struggle with wet mucus: the mucus forms a protective shell around the virus that ethanol can’t penetrate until the mucus fully dries. Once mucus is dry, sanitizers work within 30 seconds too. The practical takeaway is that soap and water is the better choice when available, especially after contact with someone who’s visibly sick.
Reducing airborne exposure is harder but not impossible. Good ventilation dilutes the concentration of virus-carrying aerosols in a room. Keeping distance from coughing or sneezing people limits your contact with larger droplets. And because people shed virus before they know they’re sick, annual flu vaccination remains the most reliable way to lower your chances of infection in the first place, training your immune system to recognize the virus before it ever reaches your airway cells.