You get the flu by breathing in tiny respiratory droplets that an infected person releases when they cough, sneeze, or even talk. These droplets can land directly in your mouth or nose, or you can inhale them into your lungs. Less commonly, you can pick up the virus by touching a contaminated surface and then touching your face. The amount of virus needed to make you sick is remarkably small, especially through airborne particles.
Droplets and Aerosols Are the Main Route
The primary way influenza spreads is through the air. When someone with the flu coughs or sneezes, they launch droplets of varying sizes into the space around them. Larger droplets tend to fall within about six feet, which is why close contact with a sick person is the biggest risk. Smaller particles, though, can linger in the air longer and travel farther, particularly in enclosed spaces with poor ventilation.
What makes influenza so efficient at spreading is how little virus it takes. Research on human volunteers has shown that the minimum infectious dose through fine aerosol particles can be as low as 0.6 to 3.0 tissue culture infectious units. In practical terms, that means inhaling even a single tiny droplet from a sick person’s breath could be enough to start an infection. This is a far lower threshold than what’s needed through other routes, which helps explain why the flu moves so quickly through offices, schools, and households.
Touching Contaminated Surfaces
Picking up the flu from a surface is less common than breathing it in, but it happens. The virus lands on doorknobs, phones, desks, and elevator buttons when a sick person touches them or coughs nearby. You then transfer the virus to your respiratory tract by touching your nose, mouth, or eyes.
How long the virus survives depends on the material. On hard, nonporous surfaces like stainless steel, influenza can remain viable for up to two weeks under lab conditions, though it loses 99% of its infectiousness within about 175 hours (roughly a week). On fabrics like cotton, the virus dies off much faster, hitting that 99% reduction in around 18 hours and remaining detectable for about one week total. So a metal handrail in a busy building poses a longer-lasting risk than, say, a cloth napkin.
What Happens Once the Virus Reaches Your Airway
Once influenza particles land on the lining of your nose, throat, or lungs, the virus latches onto sugar molecules that sit on the surface of your respiratory cells. Human flu strains are specifically adapted to recognize the version of these sugar molecules found in the human airway, which is why bird flu strains don’t typically spread easily between people (they’re built to recognize a slightly different version).
After attaching, the virus tricks the cell into pulling it inside through a process similar to how cells normally absorb nutrients. Once enclosed in a tiny bubble within the cell, the acidic environment triggers the virus to fuse with the bubble’s wall and release its genetic material. That genetic material then travels to the cell’s nucleus, hijacks the cell’s machinery, and starts churning out copies of itself. These new virus particles burst out and spread to neighboring cells, and the cycle repeats. This whole process from entry to new virus release takes only hours.
When You’re Contagious
One of the trickiest things about the flu is that you can spread it before you even know you’re sick. People typically become contagious about one day before their symptoms appear and remain contagious for five to seven days after getting sick. Children and people with weakened immune systems may shed the virus for even longer.
Not everyone who catches the flu develops obvious symptoms. A systematic review of outbreak investigations found that roughly 16% of confirmed influenza infections were asymptomatic. These people carry and shed the virus without realizing it, which makes the flu harder to contain through symptom-based precautions alone. The full extent to which asymptomatic carriers drive community spread isn’t well quantified yet, but their existence means you can catch the flu from someone who seems perfectly healthy.
Why Flu Peaks in Winter
Influenza follows a striking seasonal pattern, and the biggest factor appears to be humidity. A 30-year analysis of data across the continental United States found that drops in absolute humidity correlate most closely with rises in flu-related deaths. Low humidity affects transmission in multiple ways: it helps airborne droplets shrink and stay suspended longer, it allows the virus to survive better outside the body, and it dries out the protective mucous membranes in your nose and throat, weakening your body’s first line of defense.
Animal studies confirm this relationship directly. Guinea pig experiments showed that low temperature combined with low humidity enabled airborne flu transmission that didn’t occur in warmer, more humid conditions. This is a big part of why flu season runs from roughly October through March in the Northern Hemisphere, peaking in the coldest, driest months.
How to Reduce Your Risk
Annual vaccination remains the most effective single step for preventing the flu. It’s recommended for everyone six months and older, with particular importance for young children, older adults, pregnant women, and anyone with chronic health conditions. Household contacts of people with weakened immune systems should also be vaccinated, since they can serve as a bridge bringing the virus home.
Hand hygiene helps, though the type matters more than you might expect. A systematic review comparing soap and water with alcohol-based hand sanitizer found that sanitizer was more effective at reducing respiratory infections. In one study of childcare centers in Spain, children in a sanitizer group had a 13% lower risk of respiratory infection compared to those using soap and water alone. The difference doesn’t appear to be about how often people wash their hands, but about how effectively the method neutralizes the virus. That said, soap and water is still far better than nothing, and either option reduces risk compared to skipping hand hygiene entirely.
Beyond handwashing, practical steps include keeping distance from people who are visibly sick, avoiding touching your face in public settings, and improving ventilation in indoor spaces when possible. Since the virus survives longest on hard surfaces, wiping down shared items like phones, keyboards, and countertops during flu season is worth the effort, especially in households where someone is already sick.