Influenza spreads mainly through tiny respiratory particles that an infected person releases when they cough, sneeze, talk, or even breathe. You can also pick it up by touching a contaminated surface and then touching your eyes, nose, or mouth. Once the virus reaches your respiratory tract, symptoms typically appear about two days later, though the window ranges from one to four days.
Person-to-Person Spread
The most common way to catch the flu is simply being near someone who has it. When an infected person coughs or sneezes, they launch virus-laden droplets into the air. Larger droplets tend to land on nearby surfaces or on people within about six feet. Smaller particles, sometimes called aerosols, can linger in the air longer and travel farther, especially in poorly ventilated indoor spaces.
What makes flu particularly sneaky is the timing. Adults are contagious starting the day before any symptoms appear and remain infectious for roughly five to seven days after symptoms begin. That means someone who feels perfectly fine can pass the virus to you over lunch, and neither of you would know it at the time. Research published in PNAS estimated that about 26% of household flu transmission comes from people who never develop noticeable symptoms at all. You don’t need to be around someone who looks sick to catch the flu.
Surfaces and Hand Contact
Flu viruses survive on hard, nonporous materials like stainless steel and plastic for 24 to 48 hours. Door handles, light switches, shared keyboards, elevator buttons, and shopping carts can all harbor live virus long after an infected person touched them. Fabric and softer surfaces tend to deactivate the virus faster, but they’re not risk-free either.
The chain of events is straightforward: someone with the flu touches their nose, then touches a surface. You touch that same surface minutes or hours later, then rub your eye or eat without washing your hands. The virus enters through the mucous membranes of your eyes, nose, or mouth and begins replicating in your respiratory tract. This is why hand hygiene matters so much during flu season, even if no one around you seems sick.
Why Flu Peaks in Winter
Flu season in temperate climates runs roughly from late fall through early spring, and that’s not a coincidence. Cold, dry air plays a direct role in how efficiently the virus spreads. At lower humidity levels, the water in expelled respiratory droplets evaporates faster, shrinking them into lighter particles that stay airborne longer. At the same time, the virus itself survives better in dry conditions, remaining viable in the air for extended periods. A landmark study in PNAS confirmed that absolute humidity is a key driver: when the air is drier, more virus stays suspended and infectious for longer.
Winter behavior compounds the problem. People spend more time indoors with windows closed, reducing ventilation and increasing the concentration of airborne particles in shared spaces. Crowded holiday gatherings, schools in session, and public transit use all create ideal conditions for respiratory viruses to jump from person to person.
Animal-to-Human Transmission
Most seasonal flu circulates exclusively between humans, but influenza viruses also live in birds and pigs, and occasionally those strains cross into people. According to the World Health Organization, the primary risk factor for avian flu is direct or indirect contact with infected animals or environments contaminated by them. That includes handling live or dead poultry, working in live bird markets, and preparing infected birds for cooking in household settings.
Swine influenza viruses pose a risk to people who work in close proximity to pigs or visit locations where pigs are exhibited, like agricultural fairs. Farm workers involved in culling or managing infected animal populations face elevated exposure. These animal-origin infections are relatively rare compared to seasonal flu, but they carry public health significance because humans typically have little pre-existing immunity to them.
Who Is Most Vulnerable
Everyone can catch the flu, but certain groups face a higher risk of serious illness once infected. Young children, adults over 65, pregnant women, and people with chronic conditions like asthma, diabetes, or heart disease are more likely to develop complications. People with weakened immune systems, whether from medication or underlying illness, may also shed the virus for longer than the typical five-to-seven-day window, making them contagious for an extended period.
How to Reduce Your Risk
Annual vaccination is the single most effective tool for preventing the flu. Everyone six months and older is recommended to get a flu vaccine each season. The vaccine is reformulated yearly to match the strains expected to circulate, which is why last year’s shot doesn’t carry over.
Beyond vaccination, several everyday habits lower your chances of picking up the virus:
- Hand washing: Soap and water for at least 20 seconds, especially after being in public spaces. Alcohol-based hand sanitizer works when a sink isn’t available.
- Avoiding face touching: Your eyes, nose, and mouth are the entry points. Keeping your hands away from your face after touching shared surfaces cuts off a major transmission route.
- Improving indoor air quality: Opening windows, using air purifiers, or spending time outdoors when possible reduces the concentration of airborne virus particles in shared spaces.
- Wearing a mask: Masks reduce both the spread from an infected person and the risk of breathing in virus particles from others, particularly useful in crowded or poorly ventilated settings during peak flu season.
- Staying home when sick: The CDC recommends staying home for at least 24 hours after your symptoms are improving overall and your fever has resolved without medication.
Covering coughs and sneezes with a tissue (and discarding it immediately) prevents you from depositing virus on your hands and every surface you touch afterward. These measures work best in combination. No single step eliminates the risk entirely, but layering several of them together significantly lowers your odds of catching or spreading the flu.