Cold, Flu, & Cough Health Center

What Causes the Flu?

In order to prevent getting the flu, it is important to understand what causes the flu. 

WebMD looks at the causes of flu and how they can affect you and your family.

What Types of Viruses Cause the Flu?

The flu is actually very 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 are responsible for the large flu epidemics. Type C flu virus is more stable and usually causes milder respiratory symptoms. While the flu vaccine can help protect you from type A and B flu viruses, there is no immunization or flu shot for type C virus.

Type A flu virus is divided into different subtypes based on the chemical structure of the virus. Type B flu virus is not divided into subtypes. Both type A and type B flu viruses are responsible for the seasonal outbreaks of flu.

Type A flu viruses are found in many different animals, including ducks, chickens, pigs, whales, horses, and seals. Influenza B viruses circulate widely only among humans.

How Is the Flu Virus Spread?

The flu is a highly contagious disease. The flu virus is spread when you either inhale infected droplets in the air (spread when an infected person coughs or sneezes) or when you come in direct contact with an infected person's secretions (by kissing, touching, sharing objects such as spoons and forks). You can also transfer the flu virus to your hands by touching smooth surfaces such as doorknobs, handles, television remotes, computer keyboards, and telephones. Then when you touch your hands to your nose, eyes, or mouth, the flu virus gets absorbed.

What Is a Flu Epidemic?

Flu outbreaks are classified as epidemics, which means they occur in a set geographical area, or pandemics, which means a worldwide occurrence. According to the CDC, winter flu epidemics can cause illness in 10% to 20% of the population and are associated with 3,000 to 49,000 deaths and more than 200,000 hospitalizations per year in the United States.

Why Do People Usually Get the Flu in Winter?

Flu outbreaks occur more frequently in the winter months. Many factors may play a role in the flu's seasonal pattern including. Here's why:

• The flu virus survives for longer periods indoors in winter because the relative humidity of indoor air is very low in comparison to the outside air.
• The flu virus may stay suspended in the air for prolonged periods and thus infect others by being inhaled. The flu virus droplets can also infect by landing on sensitive body areas such as the eyes, nose, or mouth.
• In winter, humans tend to be indoors more and thus have closer contact with each other, which makes it easier for the flu virus to spread.