What Causes the Flu?

Written by the Healthline Editorial Team | Published on July 28, 2014
Medically Reviewed by George Krucik, MD, MBA on July 28, 2014

Flu: Causes

Three main types of influenza viruses cause the flu: Type A, Type B, and Type C.

All three virus types are spread in the same way: they leave an infected person’s body in droplets whenever that person coughs, sneezes, or puts their mouth on another object. The Centers for Disease Control and Prevention (CDC) report that a person with the flu can spread the virus to others up to six feet away. If you are in close contact with an infected person, you may end up inhaling infected droplets immediately.  However, you can also pick up the virus later from touching an infected object (such as a door handle or a pencil) and then touching your eyes, nose, or mouth.

Additionally, the flu is highly contagious. Most people experience flu symptoms within one to four days of getting sick. However, an infected person can begin spreading the flu virus to others one day before manifesting symptoms, and for up to a week after getting sick.

This is why it is essential to maintain vigorous personal hygiene, especially during flu season. This includes simple steps, like using antibacterial liquid to clean your hands after getting off a bus, or washing your hands extra carefully before eating. Such steps can go a long way when it comes to avoiding the flu and preventing its spread. 

Flu Virus Types

Type A

Type A influenza virus is the most virulent form of influenza and the one that causes the most severe illness. This virus often originates in populations of wild birds, poultry, or swine (including pigs and hogs), so different strains are sometimes referred to as avian (bird) or swine flu. Type A influenza is most active during colder weather in temperate climates, and its peak season typically runs from late fall to early spring.

Type A influenza virus is categorized according to variations in two proteins on the surface of the virus: H (hemagglutinin) and N (neuraminidase). According to the CDC, there are 17 subtypes of the H protein and 10 subtypes of the N protein. Different protein combinations lead to different strains of the virus. H1N1 and H3N2 are two recent strains of Type A influenza virus.

The two most active strains of Type A virus, usually H1N1 and H3N2, are usually included in each year’s flu vaccine. If a new virus strain emerges that is not covered by the annual flu vaccine, a special vaccine is made to combat the new strain of virus.

The United States government recommends that all individuals six months and older get a yearly flu vaccine.

Type B

Type B influenza is not divided into subtypes, but the virus does mutate into different strains. Unlike Type A, Type B influenza is active throughout the year. Nevertheless, it generally causes a much milder form of the flu than Type A viruses.

The strain of Type B virus most active in the population is usually included in the annual flu vaccine.

Type C

Type C influenza is the least common type of the flu, and its symptoms are generally much milder than those of Type A or B influenza. Type C influenza is not included in yearly flu vaccines.

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