You get influenza A by breathing in virus-laden particles from an infected person or by touching a contaminated surface and then touching your nose, mouth, or eyes. The virus spreads through three distinct routes, and understanding each one helps explain why flu moves so efficiently through households, schools, and workplaces, especially during winter months.
Three Routes of Transmission
Influenza A travels from person to person in three ways: aerosols, large droplets, and direct contact. These aren’t just technical distinctions. Each route delivers the virus to different parts of your respiratory system and plays a different role in how outbreaks unfold.
Aerosols are tiny particles (smaller than about 10 micrometers) that hang in the air for minutes to hours. Particles under 3 micrometers essentially never settle and can drift across a room. When you inhale these, they bypass your upper airways and land deep in your lungs, which is why aerosol transmission can lead to more severe lower respiratory infections. A single cough or even normal breathing from a sick person generates these particles.
Large droplets are the heavier spray produced by coughing, sneezing, or talking. These are bigger than 20 micrometers, settle out of the air within seconds, and generally travel no more than about 3 to 6 feet from the source. They land in your nose and throat rather than deep in your lungs. This is the classic “sneeze range” that public health guidelines focus on.
Direct contact happens when you touch respiratory secretions, either on another person or on a surface (called a fomite), and then touch your face. This is the route most under your control.
How Long the Virus Survives on Surfaces
Influenza A is surprisingly durable outside the body. On hard, non-porous surfaces like stainless steel, the virus can remain viable for up to two weeks. On fabrics like cotton, it survives about one week. The practical drop-off is faster, though: on cotton, 99% of the virus is gone within about 18 hours, while on stainless steel it takes roughly 175 hours (about a week) to reach that same 99% reduction. Microfiber falls in between at around 34 hours.
This means doorknobs, light switches, phone screens, and shared keyboards are common pickup points. Handrails on public transit, elevator buttons, and office equipment are all surfaces where the virus can linger long enough for the next person to pick it up.
What Happens When the Virus Enters Your Body
Once influenza A particles reach the cells lining your respiratory tract, the virus uses a protein on its surface to latch onto sugar molecules (sialic acid) on your cells. Each individual bond between the virus and a cell is weak, so the virus needs to form multiple connections simultaneously before it can gain entry. Once enough bonds form, the cell pulls the virus inside, and the infection cycle begins. The virus hijacks your cell’s machinery to make copies of itself, which then burst out and infect neighboring cells.
This process moves fast. Symptoms typically appear within two to three days of exposure, though they can start as quickly as one day or take up to four.
When an Infected Person Is Most Contagious
Adults with influenza A start shedding the virus about one day before any symptoms appear. This is a key reason the flu spreads so effectively: people are contagious before they know they’re sick. Viral shedding continues for roughly 5 to 7 days after symptoms start, with the highest levels of contagiousness occurring during the first 3 to 4 days of illness. People with fever are generally more infectious than those without.
Children tend to shed the virus for longer periods than adults, which partly explains why schools and daycares are such potent engines of flu transmission.
Asymptomatic Infections Spread the Virus Too
Not everyone who catches influenza A feels sick. Pooled estimates from outbreak investigations suggest about 16% of influenza infections are completely asymptomatic. These people still shed detectable virus and can pass it to others. The exact proportion of overall transmission attributable to asymptomatic carriers remains unclear, but the fact that roughly 1 in 6 infected people never develop symptoms means the virus has a built-in stealth advantage. You can catch the flu from someone who looks and feels perfectly fine.
Why Flu Spreads More in Winter
Cold, dry air creates ideal conditions for influenza A transmission. In controlled experiments using animal models, low relative humidity (20% to 35%) was most favorable for spread, while transmission was completely blocked at 80% humidity. Temperature matters too: at 5°C (41°F), transmission occurred more frequently than at 20°C (68°F), and at 30°C (86°F) no transmission was detected at all.
Indoor heating during winter months strips moisture from the air, creating exactly the low-humidity environment the virus thrives in. People also spend more time indoors in closer proximity, and cold, dry air may impair the protective mucus layer in your nasal passages, making it easier for the virus to take hold. The combination of these factors is why flu season reliably peaks in colder months.
Reducing Your Risk
Since influenza A uses multiple transmission routes, no single precaution eliminates your risk, but several measures meaningfully reduce it.
Handwashing is the most straightforward. Regular handwashing reduces respiratory infections by about 16% to 21% in the general population. That might sound modest, but applied across an entire flu season, it translates to a meaningful number of infections avoided. Soap and water for at least 20 seconds is the standard; alcohol-based sanitizers work when a sink isn’t available.
Distance matters for large droplet transmission. Staying at least 6 feet from someone who is visibly coughing or sneezing reduces your exposure to the heavier particles that settle quickly. Aerosols are harder to avoid in shared indoor spaces, which is where ventilation and air filtration play a role. Opening windows, using HEPA filters, or simply spending less time in crowded, poorly ventilated rooms during peak flu season all help.
Avoiding touching your face is simple in theory, harder in practice. The average person touches their face dozens of times per hour. Being conscious of it, especially after touching shared surfaces in public, reduces the direct contact route. Cleaning high-touch surfaces at home when someone is sick is particularly worthwhile given how long the virus survives on hard surfaces.
Annual flu vaccination remains the most effective single intervention. The vaccine’s effectiveness varies by season depending on how well it matches circulating strains, but even in lower-effectiveness years, it reduces the severity of illness and the risk of complications like pneumonia and hospitalization.