According to the Centers for Disease Control and Prevention (CDC), the flu is as an infection caused by influenza viruses. The CDC estimates that an average of 200,000 hospitalizations and 36,000 deaths in the United States are attributed to these viruses during the seasonal flu outbreak each year. There are mainly three different types of flu viruses, based on the type of proteins contained in the virus.
Influenza Type A
According to National Institutes of Allergy and Infectious Diseases, type A influenza virus is the most frightening type of flu virus, linked to the major world outbreaks or pandemics of influenza in 1918, 1957 and 1968. It typically causes the seasonal flu in the United States. Apart from human infection, it is also found in birds, dogs, whales and pigs.
Type A influenza virus is further classified into subtypes based on the two crucial surface proteins on the virus. The first protein, hemagglutinin (HA), is important for the attachment of the virus to the host cell prior to entry. The second protein, neuraminidase (NA), helps the newly formed infective virus to leave the cell. According to the NIAID, scientists have documented a total of 26 subtypes of Influenza type A. As the virus multiplies, it is able to undergo gradual changes in these surface proteins to form new strains and make it defeat the host immune system. The rationale behind yearly flu shots is to keep up with these changes, called antigenic drift. Type A is also capable of antigenic shift, resulting in a brand-new subtype capable of causing severe flu outbreaks and pandemics (global epidemics).
The swine flu, a subtype of type A (H1N1), caused a worldwide outbreak that started in 2009. It is believed to have originated in pigs, according to the NIAID.
The avian (bird) flu, another subtype of type A (H5N1), has been reported in more than a dozen countries, with about 450 serious cases in all and 60 percent case fatality. According to Flu.gov, no cases were reported in the United States as of March 2010.
Influenza type B
This type is largely found in humans and, according to the NIAID, is also involved in some cases of seasonal flu in the United States. It is thus combined with type A in the yearly flu vaccine. It also undergoes antigenic drift but rarely antigenic shift.
Influenza type C
Influenza type C causes human disease but rarely on an epidemic scale, according to the NIAID. It infects humans, dogs and pigs. The illness caused by this type is usually mild.
