You get the flu when influenza virus particles enter your nose, mouth, or eyes, almost always because someone nearby coughed, sneezed, or talked while infected. The virus travels in tiny respiratory droplets that can land directly on your face or get inhaled into your lungs. Less commonly, you pick it up by touching a contaminated surface and then touching your face. Understanding exactly how this happens, and when infected people are most contagious, can help you avoid it.
How the Virus Travels From Person to Person
The primary route is airborne droplets. When a person with the flu coughs or sneezes, they release a spray of moisture-laden particles carrying live virus. These droplets are heavy enough that most fall within about six feet, which is why close contact with a sick person is the biggest risk factor. But talking alone produces smaller droplets too, meaning you don’t need to witness a dramatic sneeze to be exposed.
Surface transmission is a secondary route. Influenza can survive on hard surfaces like doorknobs, phones, and countertops for hours. If you touch one of these surfaces and then rub your nose or eyes, the virus can establish an infection. This happens less often than direct droplet exposure, but it’s a real pathway, especially in shared spaces like offices, schools, and public transit.
What Happens Inside Your Body
Once the virus lands on the moist lining of your nose, throat, or lungs, it latches onto cells in your respiratory tract using a protein on its surface that acts like a molecular key. This protein recognizes and binds to sugar molecules on the outside of your airway cells. After locking on, the virus fuses its outer shell with the cell membrane, essentially merging with the cell and dumping its genetic material inside. The hijacked cell then starts producing copies of the virus instead of doing its normal job. Those new virus particles burst out and infect neighboring cells, and the cycle repeats rapidly.
This process triggers your immune system’s inflammatory response, which is what produces the familiar symptoms: fever, body aches, sore throat, cough, and fatigue. Uncomplicated flu typically hits the upper respiratory tract (nose and throat), and symptoms come on abruptly. Most people recover in 3 to 7 days, though cough and fatigue can linger for more than two weeks, particularly in older adults and people with chronic lung conditions.
When Infected People Are Contagious
One of the trickiest things about the flu is that people start spreading it before they know they’re sick. Most adults are infectious starting one day before symptoms appear and remain contagious for roughly 5 to 7 days after symptoms begin. That pre-symptomatic day is a major driver of transmission because the infected person feels fine and goes about their normal routine, exposing others at work, school, or home.
Children tend to shed the virus for longer than adults, which is one reason schools and daycare centers are such effective incubators for seasonal flu. And not everyone who gets infected develops noticeable symptoms. An estimated 16% of influenza infections are asymptomatic, meaning the person never feels sick but still sheds detectable virus and can pass it to others.
Why Flu Spreads More in Winter
Flu season peaks in cold, dry months, and the reasons are biological, not just behavioral. Research on influenza transmission found that low relative humidity, between 20% and 35%, is the most favorable environment for the virus to spread. At 80% humidity, transmission was completely blocked in controlled experiments. Cold temperatures amplify the effect: at 5°C (41°F), transmission rates reached 75% to 100% even at moderate humidity levels, while at 30°C (86°F), no transmission occurred at all.
Cold, dry air does two things that favor the virus. It helps viral particles stay stable and airborne longer, and it dries out the mucous membranes in your nose and throat, reducing your body’s first line of defense. Indoor heating during winter further drops humidity inside homes and offices, creating ideal conditions. This is why flu circulates year-round in tropical climates (where humidity patterns differ) but follows a sharp winter pattern in temperate regions.
Risk of Catching Flu at Home
Living with someone who has the flu puts you at significant risk, but it’s not a guarantee. Studies tracking household transmission found that the overall risk of a household contact becoming infected was about 15.7%. That means roughly one in six family members catches the flu from the sick person in their home. The odds go up with closer contact, shared bedrooms, and poor ventilation, and they’re higher for young children and older adults whose immune systems are either immature or weakened.
What Actually Reduces Your Risk
Handwashing is the simplest intervention with solid evidence behind it. A large review of over 52,000 participants found that consistent hand hygiene reduced respiratory infections by about 14%. That may sound modest, but across an entire flu season, it meaningfully lowers your chances. The key is washing with soap and water for at least 20 seconds, especially after touching shared surfaces and before eating or touching your face.
Avoiding close contact with visibly sick people is straightforward but effective, particularly during peak flu season. If someone in your household is sick, simple measures like sleeping in separate rooms, not sharing utensils, and wiping down commonly touched surfaces can reduce the chance you’ll be one of that 16% of household contacts who gets infected.
Annual flu vaccination remains the most targeted preventive tool. The vaccine’s effectiveness varies from year to year depending on how well it matches circulating strains, but even in a mismatch year, it tends to reduce the severity and duration of illness if you do get infected. Because the virus mutates each season, last year’s vaccine offers limited protection against this year’s strains, which is why it’s recommended annually.