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Scientists classify flu (influenza) viruses as influenza A, B or C based on the proteins in the virus membrane. Influenza A viruses are responsible for the majority of flu outbreaks and undergo relatively rapid genetic changes that prevent people from developing complete immunity. Influenza B causes outbreaks that affect few people and cause less severe disease than influenza A. It also changes genetically but at a slower pace. Influenza C tends to cause only minor illness and is not known to be involved in any large epidemics. The genes of influenza C are the slowest to change among flu viruses, allowing most people to develop immunity by adulthood.

In addition to the proteins in the membrane, influenza A is further classified by the surface proteins present. One of 16 varieties of hemagglutinin (H1 to H16) and one of nine varieties of neuraminidase (N1 to N9) are present on each virus particle. These proteins can occur in many possible combinations, but since 1977 most cases of human influenza A infection have been caused by H1N1, H2N2, H3N2 viruses. During years when H3N2 is the predominant flu subtype, the number of hospitalizations and deaths tend to be higher.

Flu viruses tend to occur within a single species. Most forms of human flu usually include the H1, H2 or H3 surface proteins. In recent years, types of bird flu with the H5, H7 and H9 proteins have also been transmitted to people. Some scientists believe this could result in antigenic shifts that produce a novel form of the flu that is transmissible between people. If this occurs, it could lead to a disease similar to the one that caused the 1918 flu pandemic.

When symptoms of nausea, vomiting and diarrhea occur, people sometimes refer to it as the “stomach flu.” This is a misnomer. The flu is a respiratory disease – it does not involve problems of the stomach or intestines. Although flu symptoms can include nausea, vomiting and diarrhea, these are rarely the only or predominant symptoms of the flu. Patients with these primary symptoms may actually have a stomach or gastrointestinal disorder rather than the flu.