The influenza virus is the second most common respiratory illness in the world, next to the common cold, according to 2005 information from Florida State University, with an estimate that 25 to 50 million Americans contract the flu each year. Highly contagious and most prevalent during the winter, the flu may present with muscle aches and pains, sore throat, fatigue, dry cough and runny nose, and in extreme cases, may lead to death. The influenza pathogen consists of three types.

Type A

Twenty-five different subtypes of type A influenza exist, notes the Centers for Disease Control and Prevention. The type A influenza pathogen has the ability to change its viral makeup. This process is fast and is called antigenic shift. There may be several major changes during the same flu season. The CDC notes that in 2009, a major shift produced the H1N1 influenza pandemic.

Type B

The type B influenza pathogen does not change as swiftly as the type A virus, making it easy to monitor. The CDC describes the process as antigenic drift, which produces smaller changes in the viral makeup over a longer period of time. Vaccines are constantly being updated to stay current with the pathogen's ability to change.

Type C

Type C influenza virus is different from type A and type B virus, genetically and morphologically, according to the Florida State University. Because of the resulting mild symptoms, type C is thought to be of little medical threat.