You contract the flu by inhaling virus-laden particles from an infected person’s breath, coughs, or sneezes, or by touching a contaminated surface and then touching your eyes, nose, or mouth. The virus needs to reach the moist lining of your respiratory tract to take hold, and it can get there through surprisingly varied routes. Symptoms typically appear about two days after exposure, though the window ranges from one to four days.
Respiratory Droplets and Airborne Particles
The most common way people catch the flu is by breathing in droplets that an infected person releases when they cough, sneeze, talk, or even just exhale. These droplets come in a range of sizes, and that size determines how far they travel and how deep into your lungs they can reach.
Larger droplets (bigger than about 20 micrometers across) are heavy enough to fall to the ground within seconds. They typically don’t travel more than a few feet from the person who expelled them, and if inhaled, they get trapped in the upper airways like the nose and throat. Smaller particles, under about 5 micrometers, behave very differently. They can remain suspended in the air for minutes to hours. Particles smaller than 3 micrometers essentially never settle on their own. These tiny aerosols can drift across a room and, when inhaled, penetrate deep into the lower lungs.
Here’s what makes this worse: droplets that start out large can shrink rapidly through evaporation after being expelled, turning into smaller aerosol-sized particles. A cough or sneeze generates a huge number of particles, and a large proportion of them are already under 10 micrometers. So even a brief encounter with a sick person in an enclosed space can expose you to virus that lingers in the air after they’ve left.
Surface Contact (Fomite Transmission)
When someone with the flu coughs into their hand and then touches a doorknob, counter, or phone, they can leave behind viable virus. If you touch that surface and then rub your eye or touch your nose, the virus reaches the mucous membranes where it can start an infection.
The flu virus doesn’t survive on surfaces as long as many people assume. In household testing at typical indoor conditions, live virus was recoverable from most surfaces four hours after it was deposited. By nine hours, only stainless steel still harbored detectable amounts. By 24 hours, no surface tested had viable virus remaining. Porous materials like wood lost viable virus in under four hours. So surface transmission is a real but time-limited risk, most relevant in the first few hours after contamination.
How the Virus Enters Your Cells
Once flu particles land on the lining of your nose, throat, or lungs, the virus latches onto sugar molecules called sialic acid residues that coat the surface of your cells. These sugars are found on virtually every cell membrane in your body, but the respiratory tract is where the virus has direct access. After attaching, the virus gets pulled inside the cell through a process called endocytosis, essentially tricking the cell into swallowing it. Once inside, the acidic environment triggers the virus to fuse with the cell’s own membrane and release its genetic material, hijacking the cell to produce copies of itself.
This is why the eyes, nose, and mouth are the vulnerable entry points. They provide direct access to mucous membranes rich in the receptors the virus needs. Intact skin on your hands is not a route of infection on its own.
When an Infected Person Is Contagious
One of the trickiest aspects of flu transmission is the timing. An infected person becomes contagious about one day before they feel any symptoms at all. They remain infectious for roughly five to seven days after symptoms begin, with peak contagiousness occurring in the first three to four days of illness, particularly while they have a fever. Children, people with weakened immune systems, and those who are severely ill can shed the virus for 10 days or longer.
That pre-symptomatic window matters enormously. Research modeling household transmission estimated that roughly 26% of flu spread within homes comes from people who never develop noticeable symptoms. So a significant share of infections are passed along by people who feel perfectly fine or have symptoms so mild they don’t recognize them as the flu.
Spread Within Households
Close, prolonged contact is the highest-risk scenario for catching the flu, and nothing creates more close contact than sharing a home. Once one household member is infected, the risk that any given household contact also becomes infected can reach as high as 38%. Shared bathrooms, kitchens, and common spaces create repeated opportunities for both droplet inhalation and surface contact over the course of several days.
Why Flu Peaks in Winter
If you’ve noticed that flu season reliably hits in the colder months, that’s not a coincidence or a myth. Research published in the Proceedings of the National Academy of Sciences found that absolute humidity, the total amount of water vapor in the air, explains about 90% of the variability in how long the flu virus survives outside the body and about 50% of the variability in transmission rates.
Cold winter air holds very little moisture, and heating that dry air indoors drops humidity even further. In these low-humidity conditions, the virus survives longer on surfaces and in the air. Expelled droplets also evaporate faster, shrinking into smaller aerosol particles that float longer and travel farther. The combination of longer virus survival and more efficient airborne spread creates the seasonal surge in cases that defines flu season in temperate climates.
Flu From Animals
Most flu infections come from other people, but the virus can also jump from animals to humans. Bird flu strains like H5N1 and H9N2, along with swine flu strains like H1N1 and H3N2, occasionally infect people. Horses and dogs carry their own influenza varieties as well.
These zoonotic infections almost always require direct contact with infected animals or environments contaminated with their droppings or secretions, such as live poultry markets or pig farms. Unlike seasonal flu, these animal-origin strains generally do not spread efficiently from person to person, which is why they cause sporadic cases rather than widespread outbreaks.
Reducing Your Risk
The practical steps for avoiding the flu are straightforward, though the evidence behind them is more nuanced than you might expect. Hand hygiene programs show a trend toward reducing respiratory illness, though large reviews have found the effect is modest and not always statistically significant when looking at confirmed flu specifically. The benefit is most likely in situations where surface contact is a major transmission route, like schools, daycare settings, and shared workspaces.
Large community-level trials of mask-wearing have shown little measurable difference in rates of influenza-like illness or confirmed flu compared to not wearing masks. Even comparisons between standard surgical masks and N95 respirators showed no significant difference in flu protection across multiple studies. This doesn’t necessarily mean masks can’t help in specific high-risk moments, but the population-level evidence for routine masking against flu is weak.
Vaccination remains the most broadly recommended preventive measure. Keeping distance from visibly sick people, improving ventilation in indoor spaces, and avoiding touching your face after contact with shared surfaces all reduce the opportunities for the virus to reach your respiratory tract. Given that the virus survives only a few hours on most household surfaces, regular cleaning of high-touch areas like light switches, faucet handles, and phones during flu season can meaningfully cut one transmission route.