Influenza, known as the flu, is a common virus that causes illness in ducks, chickens, whales, horses, pigs, seals and humans, according to the Centers for Disease Control and Prevention (CDC). Influenza is technically part of the orthomyxovirus family. This virus can transform, in a process called a "antigenic drift," into new strains after infecting the host. The virus also has the ability to jump from one species to another. This host shift creates difficulties in developing vaccines to protect humans and animals. Scientists have named three main types of the influenza viruses, A, B and C, all identified by the internal antigens (types of proteins) of the viruses.
Influenza A
The influenza A virus is one of the most common flu viruses, accounting for nearly 95 percent of flu infections each year, according to the Merck Manuals. Scientists classify the influenza A virus into subtypes for research purposes. This typing is based on a protein analysis of the composition of the virus surface. Hemagglutinin (identified as "H," sometimes also labeled "HA") and neruaminidase ("N" or "NA") are easily identified by researchers studying the surface composition of the the influenza A virus. The two proteins also have various subtypes, or strains, and this identification is critical to making yearly vaccines. Protein H is divided into 16 subtypes, with N characterized by nine different subtypes. Influenza A is labeled each year using this surface composition identification.
The 2009 virus (popularly known as Swine Flu, due to the shift from pigs to humans) was identified as hemagglutinin protein subtype 1 and subtype 1 of the neruaminidase protein. When the year of infection designation is added to name, it is identified as "N1H1-2009." The scientific community also identifies the place of flu origin in the name, if this identification is possible. For example, the widespread epidemic of 1957 was known as the H2N2 Asian Flu. Normal infectious strains include H1N1, H1N2 and H3N2. The H2N2 virus created seasonal epidemics from 1957 through 1968, but it is not currently active.
Influenza B
The Influenza B virus also contributes epidemics each year, according to the Mayo Clinic, but these are "smaller, more localized outbreaks." While B viruses are not categorized into subtypes, they are divided into strain types. The Pennsylvania Pandemic Influenza Preparedness Planning Summit reported in 2006 that the influenza B virus does not shift from animals to humans and that the composition proteins of the virus itself "change only by the more gradual process of antigenic drift."
Influenza C
Influenza C does cause illness, but scientists classify the types of illness as "mild respiratory," according to the CDC. Since this virus is not linked to epidemics, the medical and scientific funding is focused on research of the other influenza viruses. Yearly vaccines are not developed for influenza C, and the virus is not classified into subtypes, since it is fairly stable and not prone to antigenic drift.
