What Causes the Flu?

Medically Reviewed by Sabrina Felson, MD on September 14, 2023
2 min read

Want to avoid getting the flu? A good start is to know what causes it.

This illness is different from a cold. While more than 100 different viruses can cause a cold, only influenza virus types A, B, and C cause the flu.

Type A and B viruses cause the large seasonal outbreaks. Type C usually causes milder respiratory symptoms. While the flu vaccine can help protect you from types A and B, there is no immunization for type C virus.

Type A flu viruses are also found in many different animals, including ducks, chickens, pigs, whales, horses, and seals. Type B viruses only affect people.

The flu is a highly contagious disease. It spreads when you come into contact with the stuff someone else sneezes or coughs up.

You could breathe it in, or get it from kissing. You could get it on your hands from objects like silverware, doorknobs, handles, television remotes, computer keyboards, and telephones.

The virus enters your body when you touch your hands to your nose, eyes, or mouth.

Outbreaks are grouped in two ways:

  • Epidemics happen in a set area.
  • Pandemics take place on a worldwide scale.

The CDC says in the winter of 2021-22, some 9 million Americans came down with the flu, 10,000 were hospitalized, and 5,000 died of the disease. 



Many things may play a role:

  • The virus lives longer indoors in winter, because the air is less humid than outside.
  • While it’s alive and in the air, it’s easy for people to inhale it, or for it to land on the eyes, nose, or mouth.
  • We spend more time indoors and have closer contact with each other, which makes it easier for the virus to spread.

You can spread the flu 7 days after symptoms start. The virus can live in your mucus and spit up to 24 hours before you start to feel bad. This means you might give it to someone else a full day before your symptoms begin.

Young children can still spread the flu even into the second week of illness.

Keep your hands clean. Wash them often to remove germs. Try to avoid touching your face. Wear a mask. And get a flu shot. The CDC makes a flu vaccine based on the type A strain that it believes will be most prevalent in the coming flu season. This is what you get with the annual flu shot or FluMist nasal spray. 

Also see WebMD's "What Is FluMist?"