Influenza B spreads primarily through respiratory droplets released when an infected person coughs, sneezes, talks, or breathes. These droplets can land in the mouths or noses of people nearby, or be inhaled into the lungs. You can also pick up the virus by touching a contaminated surface and then touching your face. Understanding exactly how this works, and how long the virus stays active in different environments, can help you reduce your risk during flu season.
Respiratory Droplets and Airborne Spread
When someone with influenza B coughs or sneezes, they release a spray of tiny droplets containing live virus. Larger droplets tend to fall within about six feet of the person, which is why close contact is the biggest risk factor. But smaller particles can linger in the air longer, especially in enclosed, poorly ventilated spaces like classrooms, offices, or public transit. Simply being in the same room as someone who is actively ill can be enough for transmission, particularly if the air isn’t circulating well.
This is why outbreaks tend to cluster in places where people spend extended time together indoors: schools, nursing homes, workplaces, and households. The virus doesn’t need dramatic coughing fits to spread. Normal conversation and breathing release smaller quantities of viral particles that can still infect others.
Surface Contact and Hand-to-Face Transmission
Influenza B also spreads through what’s called fomite transmission. An infected person touches their nose or mouth, then touches a doorknob, phone, keyboard, or countertop. The next person picks up the virus on their hands and transfers it to their own eyes, nose, or mouth.
Research published in the Journal of Infectious Diseases found that both influenza A and B viruses survive 24 to 48 hours on hard, nonporous surfaces like stainless steel and plastic. On softer, porous materials like cloth, paper, and tissues, the virus lasts less than 8 to 12 hours. This means high-touch surfaces in shared spaces, think light switches, elevator buttons, and shared office equipment, can remain infectious for up to two days if not cleaned.
When You’re Contagious
One of the trickiest aspects of influenza B is that you become contagious before you feel sick. Most adults start shedding the virus in their upper respiratory tract about one day before symptoms appear. That means you can spread the flu to coworkers, family members, or classmates while you still feel perfectly fine.
Once symptoms start, adults typically remain infectious for about 5 to 7 days. Children, people with weakened immune systems, and those who are severely ill can shed the virus for 10 days or more after symptoms begin. Young children in particular tend to shed higher amounts of virus, which is one reason schools and daycares are such effective incubators for seasonal flu outbreaks.
Viral shedding tends to peak in the first two to three days of illness, which is when you’re most likely to pass the virus to someone else. Even as you start feeling better toward the end of that window, you may still be releasing enough virus to infect others.
Why Cold, Dry Weather Helps the Virus Spread
Flu season hits hardest in winter, and that’s not a coincidence. Research published in PLOS Pathogens demonstrated that influenza transmission is strongly influenced by both temperature and humidity. Low relative humidity, between 20% and 35%, created the most favorable conditions for the virus to spread. At 80% relative humidity, transmission was completely blocked in experimental settings.
Temperature plays an equally important role. At 5°C (41°F), transmission occurred more frequently than at 20°C (68°F). At 30°C (86°F), no transmission was detected at all. Cold, dry air does two things: it helps viral particles stay suspended and infectious in the air longer, and it dries out the mucous membranes in your nose and throat, reducing your body’s natural defense against inhaled pathogens.
This combination of cold temperatures and low indoor humidity (since heating systems dry out the air) is precisely what makes winter months so favorable for influenza B transmission in temperate climates.
How Effective Is Hand Hygiene?
Alcohol-based hand sanitizers work against influenza, but with an important caveat. When the virus is in a thin liquid like saline, ethanol-based sanitizers inactivate it completely within 30 seconds. However, influenza virus suspended in actual respiratory mucus, the thick, sticky kind you’d encounter in real life, is much harder to kill.
A study from the American Society for Microbiology found that the virus embedded in mucus remained active even after two full minutes of rubbing with an 80% ethanol-based sanitizer. Complete inactivation in mucus took roughly four minutes, about eight times longer than in saline. This matters because when someone sneezes onto their hand or a surface, the virus sits inside a protective layer of mucus, not in a neat, easily disinfected solution.
Handwashing with soap and water is generally more effective when your hands are visibly contaminated, because the physical scrubbing and rinsing action helps break down and remove the mucus along with the virus. If you’re relying on hand sanitizer, apply a generous amount and rub thoroughly for longer than you think is necessary.
Reducing Your Risk of Infection
The most practical steps for avoiding influenza B line up with how the virus actually spreads. Keep distance from people who are visibly ill, especially during the first few days of their symptoms when shedding peaks. In indoor environments during flu season, ventilation matters: opening windows or using air purifiers with HEPA filters can reduce the concentration of airborne viral particles.
Clean high-touch surfaces regularly with standard disinfectants, paying particular attention to shared items in kitchens, bathrooms, and workspaces. Since the virus survives up to two days on hard surfaces, a daily wipe-down of commonly touched objects is a reasonable precaution during active flu season.
Avoid touching your face, particularly your eyes, nose, and mouth, when you’ve been in public spaces. Wash your hands with soap and water for at least 20 seconds, especially after being in crowded settings. And if you’re the one who’s sick, staying home for at least 24 hours after your fever resolves helps prevent spreading the virus during your most infectious period. Annual flu vaccination remains the single most effective tool for preventing influenza B infection, as the vaccine is designed each year to target the specific strains expected to circulate.