Influenza (flu) is a respiratory infection in mammals and birds. It is caused by an RNA virus in the family Orthomyxoviridae. Influenza virus is divided into four main types (Influenza A, Influenza B, Influenza C, Influenza D), which are distinguished by differences in two major internal proteins (hemagglutinin (HA) and neuraminidase (NA)). Three of the four types of influenza viruses affect humans: Type A, Type B, and Type C. Type D has not been known to infect humans, but is believed to have the potential to do so. Influenza virus type A is found in a wide variety of bird and mammal species and can undergo major shifts in immunological properties. Influenza virus type B is largely confined to humans and is an important cause of morbidity. Little is known about Influenza virus type C, which is not an important source of morbidity. Influenza D was identified in 2016.
Influenza A virus is further divided into subtypes based on differences in the membrane proteins hemagglutinin (HA) and neuraminidase (NA), which are the most important targets for the immune system. The notation HhNn is used to refer to the subtype comprising the hth discovered Hemagglutinin (HA) protein and the nth discovered neuraminidase (NA) protein. The influenza viral Hemagglutinin (HA) protein is a homo trimer with a receptor binding pocket on the globular head of each monomer, and the influenza viral neuraminidase (NA) protein is a tetramer with an enzyme active site on the head of each monomer. Subtypes are further divided into strains; each genetically distinct virus isolate is usually considered to be a separate strain.