The influenza virus is divided into three main types titled type A, type B and type C, says the Centers for Disease Control and Prevention (CDC). Humans can be infected by all three types of influenza. Type A influenza is divided into multiple subtypes, including H1N1, H1N2 and H3N2 viruses.
Influenza A and B are most often responsible for the seasonal flu that affects many people throughout the flu season, which begins in late fall and ends mid-spring. Most of the influenza viruses cause the same symptoms. The classification system is used to identify where the strain originated, the year of discovery and other characteristic information of the virus.
Type A
Influenza type A can infect humans, birds, horses, pigs and other animals, but the CDC says wild birds are the natural hosts of the viruses. Type A influenza is the only one that is divided into subtypes and the only one that is known to infect birds. Symptoms of influenza A include fever, body aches, vomiting, nausea and several respiratory symptoms. Coughing, sneezing, nasal drainage, congestion and sometimes wheezing are associated with some strains of the flu. The University of Maryland Medical Center points out that influenza type A is most often responsible for hospitalizations and is most predominant during the typical flu season.
Type B
Influenza type B is thought to only infect humans. Type B viruses do not have subtypes. Influenza type B viruses can cause death in humans but are generally considered to be less severe than type A viruses. A human epidemic is possible with type B viruses, but not a pandemic. For example, an influenza type B epidemic may affect the majority of a community and may even spread to other communities. A pandemic can spread world-wide, causing more cases than previously anticipated by health professionals.
Type C
Type C influenza viruses are the mildest of the flu types. These are not responsible for epidemics or pandemics and do not have subtypes. Like the other flu viruses, type C influenza causes many of the common flu symptoms.
