The flu spreads mainly through tiny virus-laden particles that an infected person releases into the air by coughing, sneezing, talking, or even breathing. These particles can be inhaled by people nearby, or they can land on surfaces where the virus survives long enough to be picked up by hand and transferred to the mouth, nose, or eyes. What makes flu particularly hard to contain is that a person becomes contagious about a day before they feel sick, meaning they can spread the virus without knowing they have it.
Respiratory Droplets and Aerosols
When someone with the flu coughs or sneezes, they release a burst of particles in a wide range of sizes. The larger droplets, roughly 20 micrometers or bigger, are heavy enough to fall to the ground within seconds. These are the ones that land on nearby people or surfaces within a few feet. But a large number of the particles produced by a cough or sneeze are much smaller, under 5 to 10 micrometers in diameter, and they behave very differently.
These tiny particles, sometimes called aerosols or droplet nuclei, are light enough to float in the air for extended periods. A 5-micrometer particle takes about an hour to fall just 3 meters. Particles smaller than 3 micrometers essentially never settle at all. Because they stay suspended, air currents can carry them across a room or through a ventilation system, creating the potential for infection well beyond arm’s length from the sick person. Once inhaled, these fine particles can penetrate deep into the lungs, while larger droplets tend to get trapped in the nose and throat.
Even after being expelled, droplets shrink rapidly through evaporation. A droplet that started out too large to stay airborne can lose enough water to become a tiny droplet nucleus that floats instead of falling. This means the actual number of airborne, inhalable particles is higher than what someone initially exhales.
Surface Contact and Hand-to-Face Transfer
The flu virus doesn’t just travel through the air. It can land on surfaces and remain infectious long enough for someone else to pick it up. On hard, nonporous surfaces like stainless steel, measurable amounts of influenza A can transfer to hands for up to 24 hours after contamination. The virus survives for a much shorter time on soft or porous materials: on paper tissues, for example, the transfer window is only about 15 minutes.
The practical risk from contaminated surfaces depends on the amount of virus deposited. When a person shedding large quantities of the virus touches a stainless steel surface, that surface can remain a transmission risk for roughly 2 to 8 hours. From there, the chain of infection is straightforward: you touch a contaminated doorknob, phone, or countertop, then touch your face. The virus enters through the mucous membranes of your eyes, nose, or mouth.
When You’re Most Contagious
The flu has an incubation period of about two days on average, though it can range from one to four days after the virus first infects the respiratory tract. Viral shedding, the process of releasing infectious virus, begins within 24 hours before symptoms appear. This means you can be spreading the flu before you even realize you’re sick.
Once symptoms set in, viral shedding ramps up fast. It peaks during the first one to two days of illness, stays elevated for another day or two while symptoms are most severe, then drops off. Most adults remain infectious from about one day before symptoms start to roughly five to seven days afterward. The window of greatest infectiousness is during the first three to four days of illness, especially if you have a fever.
Children, people with weakened immune systems, and those who are severely ill can shed the virus for 10 days or more after symptoms begin. And roughly 36% of flu infections are completely asymptomatic, meaning the person never develops noticeable symptoms. These silent infections are estimated to be responsible for about 26% of household transmission, a significant share of spread that’s essentially invisible.
Why Flu Spreads More in Winter
Flu season reliably peaks in the colder months, and the primary reason comes down to moisture in the air. 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 50% of the variation in flu transmission efficiency and 90% of the variation in how long the virus survives outside the body. Relative humidity, the percentage you see on weather reports, is a much weaker predictor.
Cold winter air holds far less moisture than warm summer air, even indoors where heating systems dry the air further. In these low-humidity conditions, two things happen that favor the virus. First, exhaled droplets evaporate faster, shrinking into smaller particles that stay airborne longer and travel farther. Second, the virus itself remains viable for a longer period when surrounding air contains less water vapor. The combination of longer-floating, longer-lasting virus particles creates ideal conditions for transmission. One hypothesis is that higher humidity levels degrade the fatty outer coating that protects influenza, inactivating it faster.
Reducing Your Risk of Catching or Spreading Flu
Because the flu travels through the air and on surfaces, blocking both routes matters. A University of Michigan study found that combining face masks with alcohol-based hand sanitizer reduced flu-like illness rates by 10 to 50% over a flu season, with the upper end of that range seen when both measures were used consistently together.
Hand hygiene targets the surface route. Washing your hands or using sanitizer after touching shared surfaces, especially in public spaces during flu season, interrupts the chain from contaminated surface to face. Masks target the airborne route, both by filtering particles you might inhale and by capturing droplets you exhale if you’re infected.
Ventilation also plays a role, particularly in indoor spaces where aerosols can accumulate. Opening windows, using air purifiers, or simply spending less time in crowded, poorly ventilated rooms during peak flu season all reduce exposure. Since the virus is most concentrated during the first few days of illness, staying home while symptomatic, and especially while feverish, prevents the highest-risk period of shedding from happening in shared spaces.