The influenza virus causes a respiratory disease often called "the flu." The New York State Health Department reports that more than 200,000 people infected with the flu are hospitalized every year, and about 36,000 people die from the flu every year. There are three main types of influenza virus that differ based on geographical location and rate of spread.
Influenza A
According to the Centers for Disease Control and Prevention, influenza A causes seasonal epidemics annually in the United States, usually during the winter season. Influenza A is further classified based on the types of two proteins--hemagglutinin (H) and neuraminidase (N). The CDC explains that there are 16 known types of H and nine types of N. Strains of influenza A that are present in the U.S. population include H1N1 and H3N2. The National High Magnetic Field Laboratory, associated with Florida State University, describes influenza A as a virus that can cause epidemics and pandemics, and can affect humans as well as other animal species. Because influenza A can cause such widespread infections, it is included in the yearly influenza vaccine.
Influenza B
In contrast to influenza A, influenza B infects only humans, according to the National High Magnetic Field Laboratory. Influenza B does not generally cause widespread infectious epidemics or pandemics, but it is often responsible for localized influenza outbreaks. Like influenza A, influenza B is included in the seasonal flu vaccines.
Influenza C
Influenza C is quite different at the molecular level from influenza A and B, and does not cause flu symptoms like influenza A and B. The National High Magnetic Field Laboratory reports that influenza type C is "of little medical concern."
