You get the flu by inhaling virus-carrying particles from an infected person or by touching a contaminated surface and then touching your eyes, nose, or mouth. The influenza virus spreads through three main routes: airborne droplets, fine aerosol particles, and direct contact with contaminated surfaces. These routes aren’t mutually exclusive, and in most real-world situations, more than one is at play.
Respiratory Droplets and Aerosols
When someone with the flu coughs, sneezes, or even talks, they release a spray of respiratory particles in different sizes. Larger droplets (bigger than 5 micrometers) are heavy enough that gravity pulls them down quickly. They travel ballistically through the air and land on nearby surfaces or mucous membranes within about 3 feet. This is why close contact with a sick person is the highest-risk scenario.
Smaller particles, true aerosols, can float in the air much longer and travel farther. These fine particles are especially relevant in poorly ventilated indoor spaces where they accumulate over time. You don’t need to be standing right next to someone to inhale them. A shared office, classroom, or airplane cabin with stale air creates the right conditions for aerosol exposure, even if the sick person is across the room.
Contaminated Surfaces
The flu virus can survive on surfaces for surprisingly long periods. On hard, non-porous materials like stainless steel, viable virus has been detected for up to two weeks. On fabrics like cotton, the virus remains infectious for roughly a week, though the amount drops quickly. On cotton, 99% of the virus is gone within about 18 hours. On stainless steel, that same 99% reduction takes roughly 175 hours, or about a full week.
This means doorknobs, light switches, phones, elevator buttons, and shared desks can all serve as transfer points. You pick up the virus on your fingers, then introduce it to your body the next time you rub your eye or touch your nose. Frequent handwashing matters precisely because of this route.
What the Virus Does Once It Reaches Your Airway
Once influenza particles land in your respiratory tract, the virus has to get past your body’s first line of defense: the thick layer of mucus coating your airways. The virus carries a surface protein that acts like molecular scissors, cutting through the sugary molecules on mucus, cilia, and the protective coating on your airway cells. This clears a path so the virus can reach the actual cell surface underneath.
Once it locks onto a target cell, the virus triggers the cell to pull it inside through a normal cellular intake process. From there, the virus hijacks the cell’s machinery. Its genetic material travels to the cell nucleus, where it forces the cell to produce copies of viral components. New virus particles assemble at the cell surface and bud off, ready to infect neighboring cells. The same molecular scissors that helped the virus get in now help newly made copies break free from the dying host cell and avoid clumping together, so they can spread efficiently to fresh targets.
The Contagious Window
The incubation period for flu is typically 1 to 4 days after exposure, meaning you won’t feel anything at first. But here’s the critical detail: most adults become infectious the day before symptoms appear. You’re already spreading the virus before you know you’re sick. Peak contagiousness hits during the first 3 to 4 days after symptoms start and is highest when you have a fever. Viral shedding from the upper respiratory tract generally continues for 5 to 7 days after symptoms begin.
Spread From People Who Never Feel Sick
Not everyone who catches the flu develops noticeable symptoms. Roughly 36% of influenza infections are asymptomatic, based on household transmission studies published in PNAS. These silent carriers are less infectious than people with obvious symptoms, transmitting virus at about 57% the rate of symptomatic cases. Still, asymptomatic individuals are estimated to account for about 26% of all household transmission. That’s a meaningful share of spread coming from people who have no reason to suspect they’re carrying the virus.
Why Flu Peaks in Winter
Flu season follows a predictable pattern, and humidity is the key driver. Research has shown that absolute humidity, the total amount of water vapor in the air, explains about 90% of the variation in how long the flu virus survives outside the body and about 50% of the variation in transmission rates. Relative humidity (the percentage you see on weather apps) is a much weaker predictor.
In winter, cold air holds very little moisture. Both outdoor air and heated indoor air become extremely dry. This low absolute humidity helps the virus survive longer on surfaces and in airborne particles, making transmission more efficient. It’s not the cold itself that causes flu season. It’s the dry air that comes with it. This also helps explain why spending more time indoors in winter compounds the problem: you’re breathing recirculated dry air in closer quarters with other people.
Who Is Most Vulnerable to Infection
Anyone can catch the flu, but certain people face a harder time fighting it off. Those with chronic lung conditions like COPD or cystic fibrosis have airways that are already compromised, making it easier for the virus to take hold and harder for the body to clear it. People with disabilities affecting muscle function, lung capacity, or the ability to cough and swallow effectively are also at higher risk because their natural airway-clearing mechanisms are weakened. Young children, older adults, and people with suppressed immune systems round out the high-risk groups, not necessarily because they catch the virus more easily, but because their bodies are less equipped to contain it once infection starts.
Practical Ways Transmission Happens
Knowing the biology is useful, but it helps to picture the everyday scenarios where flu actually spreads. A coworker who feels “a little off” but comes in anyway is shedding virus with every cough and conversation. The shopping cart handle at the grocery store, the pen at the bank counter, the handrail on the bus: all potential fomite transfer points. A family dinner where someone is in their first day of symptoms, sitting across the table, puts everyone within droplet range.
Children are especially efficient spreaders. They shed higher amounts of virus for longer periods than adults, they’re less consistent about covering coughs, and they share spaces (classrooms, daycare) where close contact is constant. A child bringing the flu home from school is one of the most common ways it enters a household.
The combination of a one-day presymptomatic contagious window, a large percentage of silent infections, and a virus that can persist on hard surfaces for days makes influenza remarkably effective at moving through populations. By the time you realize someone near you is sick, you may have already been exposed for 24 hours or more.