You get the flu by breathing in virus-laden particles released by an infected person, or by touching a contaminated surface and then touching your nose, mouth, or eyes. Most transmission happens through the air at close range, but the virus can also travel surprising distances indoors and spread from people who don’t look sick at all.
Breathing It In: The Main Route
When someone with the flu coughs, sneezes, talks, or even just breathes, they release a cloud of tiny particles into the air. Many of these particles are smaller than 5 micrometers, small enough to stay suspended in the air for over an hour. Particles under 3 micrometers essentially never settle. These fine aerosols can travel 7 to 8 meters from the person who expelled them, which means you don’t always need to be standing right next to someone to be exposed.
Larger droplets, the visible spray from a sneeze, tend to fall to the ground within seconds. But the smaller particles, called droplet nuclei, shrink further through evaporation after they’re expelled, making them even lighter and more persistent. These are the particles that penetrate deep into your lower respiratory tract when inhaled, which is why indoor spaces with poor ventilation are especially risky. Outbreaks hit hardest in crowded, poorly ventilated settings where aerosols accumulate rather than disperse.
Surface Contact and Touch
The flu virus can also land on surfaces like doorknobs, phones, elevator buttons, and shared keyboards. If you touch one of these surfaces and then touch your face, the virus can enter through the mucous membranes of your nose or mouth. There’s also evidence that the virus can use the eyes as a portal of entry, though this route is more strongly associated with certain avian influenza strains. Still, it’s a reminder that rubbing your eyes with unwashed hands is a real transmission risk.
When Infected People Are Most Contagious
A person with the flu becomes contagious about one day before their symptoms appear. That means someone can spread the virus before they even know they’re sick. Once symptoms start, the highest level of virus shedding happens during the first three to four days of illness, especially while a person has a fever. Most adults remain infectious for roughly five to seven days after symptoms begin, though young children and people with weakened immune systems can shed the virus for longer.
This pre-symptomatic window is one reason the flu spreads so effectively. You can catch it from a coworker, classmate, or family member who feels perfectly fine at the time.
People Without Symptoms Spread It Too
Not everyone who catches the flu gets noticeably sick. About 36% of influenza infections are completely asymptomatic, meaning the person never develops a cough, fever, or body aches. These silent infections aren’t harmless from a public health standpoint: asymptomatic cases are estimated to account for roughly 26% of all household transmission. They are somewhat less infectious than symptomatic cases, roughly 57% as infectious on average, but because these individuals don’t know they’re infected, they don’t take precautions and continue their normal routines.
Why Flu Spreads More in Winter
Flu season peaks in colder months, and the reason comes down to moisture in the air. Research published in PNAS found 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 once airborne. When the air is cold and dry, the virus stays viable much longer outside the body. Absolute humidity is a far stronger predictor of transmission rates than either temperature or relative humidity on their own.
The mechanism appears to be about viral survival rather than droplet physics. Scientists initially suspected that dry air simply helped create more tiny, floating droplet nuclei. But the data points to something more direct: when absolute humidity is low, the virus itself remains infectious for a longer period while suspended in the air. This gives it a wider window to reach a new host. Indoor heating during winter makes the air even drier, compounding the effect.
Catching It From Animals
Most people catch the flu from other people, but the virus can also jump from animals to humans. Avian influenza (bird flu) primarily infects people who have direct or indirect contact with infected birds or contaminated environments. The biggest risk comes from handling live or dead infected poultry, working in live bird markets, and participating in culling operations on poultry farms. Properly cooked poultry and eggs do not transmit bird flu.
Swine flu spreads to humans through close proximity to infected pigs, particularly at livestock farms and locations where pigs are exhibited, like agricultural fairs. These zoonotic strains don’t typically spread efficiently from person to person, but when they do adapt, they can trigger pandemics, as happened in 2009 with the H1N1 swine flu.
The Timeline From Exposure to Illness
After the virus enters your body, you won’t feel anything right away. The incubation period, the gap between exposure and the first symptoms, averages about two days but can range from one to four days. During the tail end of this window, you’re already contagious. So from the moment the virus reaches your respiratory tract, it takes roughly 24 to 96 hours before the familiar onslaught of fever, chills, muscle aches, and fatigue sets in.
Practical Ways to Lower Your Risk
Since the flu travels primarily through the air, ventilation matters. Opening windows, using air purifiers, or simply avoiding prolonged time in stuffy, crowded rooms during flu season all reduce your exposure to lingering aerosols. Washing your hands frequently handles the surface-contact route, and keeping your hands away from your face limits the virus’s access to your mucous membranes.
Annual flu vaccination remains the most effective way to reduce your chances of infection and, if you do get sick, to blunt the severity. Because infected people are contagious before symptoms appear and more than a third of cases are completely silent, you can’t rely on avoiding visibly sick people alone. The virus circulates through seemingly healthy individuals in offices, schools, and homes every flu season.