You get influenza B the same way you get most respiratory viruses: by breathing in tiny droplets that an infected person releases when they cough, sneeze, or talk. The virus can also spread when you touch a contaminated surface and then touch your mouth, nose, or eyes. Unlike influenza A, which circulates in birds, pigs, and other animals, influenza B exists almost exclusively in humans, meaning you can only catch it from another person.
How the Virus Travels Between People
The primary route is airborne droplets. When someone with the flu coughs or sneezes, they spray virus-laden particles into the air. These droplets can land directly in your mouth or nose, or you can inhale them into your lungs. Close contact, roughly within about six feet, carries the highest risk because larger droplets fall to the ground quickly. Smaller aerosol particles can linger in the air somewhat longer, especially in poorly ventilated indoor spaces.
Surface contact is the secondary route. Influenza B can survive on hard surfaces like doorknobs, phones, and countertops for hours. If you touch one of these surfaces and then touch your face, you give the virus a direct path to your respiratory tract. This is why hand washing is one of the simplest ways to cut your risk.
When an Infected Person Is Contagious
One of the trickiest things about influenza B is that people become contagious before they feel sick. Viral shedding, the period when the virus actively leaves your body, typically begins about one day before symptoms appear. That means someone sitting next to you at work or school can spread the virus without knowing they’re infected.
Most adults remain contagious for roughly five to seven days after symptoms start. Children, people with weakened immune systems, and those who are severely ill can shed the virus for 10 days or longer. The incubation period, the gap between exposure and your first symptoms, is usually one to four days.
Why Influenza B Peaks in Winter
Flu season follows a predictable pattern, and humidity plays a surprisingly large role. Research published in the Proceedings of the National Academy of Sciences found that absolute humidity, the total amount of moisture in the air, explains about 90% of the variation in how long influenza survives outside the body. When the air is cold and dry, the virus stays intact and airborne longer. When humidity rises, viral particles absorb moisture, grow heavier, and drop out of the air faster.
This relationship is strongly nonlinear, meaning small drops in humidity during winter can cause outsized jumps in viral survival. It’s a big reason why flu transmission surges between November and March in the Northern Hemisphere, and why spending more time indoors with recirculated dry air compounds the problem.
What Happens Inside Your Body
Once influenza B particles reach the lining of your upper respiratory tract, the virus latches onto sugar molecules on the surface of your airway cells. After binding, the virus is pulled inside the cell, hijacks its machinery to make copies of itself, and then bursts out to infect neighboring cells. This rapid replication triggers the immune response responsible for the fever, body aches, and fatigue that define the flu.
The infection typically stays concentrated in the respiratory tract, which is why sore throat, cough, and nasal congestion are hallmark symptoms. In some cases, particularly in young children or older adults, the virus can move deeper into the lungs and cause complications like pneumonia.
Why Influenza B Only Comes From Humans
Influenza A has a vast animal reservoir. It circulates in wild birds, poultry, pigs, and other species, which is why new pandemic strains can emerge when animal and human viruses swap genetic material. Influenza B is fundamentally different. It circulates almost exclusively in humans. There have been isolated detections in seals and horses, and antibodies to influenza B have been found in dogs, but none of these animals serve as a meaningful source of human infection.
This limited host range has a practical consequence: influenza B evolves more slowly than influenza A and doesn’t produce pandemic strains. It does, however, cause significant seasonal illness, and it tends to hit children and young adults particularly hard.
Current Circulating Strains
Influenza B historically split into two lineages called Victoria and Yamagata. For years, seasonal vaccines included protection against both. That changed after the COVID-19 pandemic. No confirmed Yamagata lineage viruses have appeared in global surveillance since March 2020, and both the World Health Organization and the FDA’s vaccine advisory committee have recommended dropping Yamagata from influenza vaccines. Today, the Victoria lineage is the only influenza B strain circulating in humans.
How to Lower Your Risk
Because influenza B spreads through droplets and contaminated surfaces, prevention comes down to a few practical habits. Wash your hands frequently with soap and water, especially after being in public spaces. Avoid touching your face when your hands aren’t clean. Keep distance from people who are visibly coughing or sneezing when possible.
Annual flu vaccination remains the most effective single step. Each season’s vaccine is updated to match circulating strains, including the current Victoria lineage of influenza B. Vaccination won’t guarantee you avoid infection, but it significantly reduces the odds of severe illness, hospitalization, and complications. If you do get sick, staying home for at least 24 hours after your fever breaks helps protect the people around you during your most contagious window.