The Influenza Virus

The flu is caused by three main types of influenza virus: Type A, Type B, and Type C:

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, 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). There are 16 subtypes of the H protein and nine 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 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. This is the reason people should get an annual influenza vaccine and a special vaccine for the H1N1 influenza virus.

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 vaccines.

How The Flu Spreads

All three flu viruses 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. If you’re in close contact with an infected person, you may end up inhaling infected droplets immediately, but you can also pick up the virus later from touching an infected object like a door handle or a pencil, and then touching your eyes, nose, or mouth.

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