The flu is a common illness caused by influenza viruses. It leads to symptoms of congestion, runny nose, coughing, fatigue and headache for one to two weeks, according to the Nemours Foundation. When an influenza virus infects a person, the immune system sets off a cascade of events designed to fight the flu but the rapidly changing virus ensures that new strains are continually available to spread the next flu season.
Types
Three types of flu viruses, named A, B and C, exist in nature, according to the Centers for Disease Control and Prevention (CDC). Proteins on the surface of the virus define the three types. Type A is further broken down into subtypes and strains created when the virus mutates and recombines. Types A and B circulate around the globe each year, often causing epidemics of the flu during the wintertime. Type A is the most common, with subtypes and strains of Type A influenza becoming predominant in any given year.
Naming
Type A influenza is named by identifying the specific proteins present in the virus. The two main proteins on the viral surface are hemagglutinin, referred to as H, and neuraminidase, known as N, according to the CDC. The 16 types of H protein and nine types of N protein are each given a number and an influenza virus is given the name of the particular H and N it carries. So influenza A(H3N2), a prominent strain in the 2009 flu season, has hemagglutinin No. 3 and neuraminidase No. 2 on its surface.
Origin
Flu viruses have existed in nature for millions of years, frequently carried by wild birds, according to the National Institute of Allergy and Infectious Disease. Since influenza mutates frequently and rapidly, it often changes to a form that can infect other species. Typically, it travels from wild birds to domesticated ducks and chickens, then on to pigs and from there becomes a strain that can infect humans.
Recombination and Mutation
An influenza virus changes its structure using mutation and recombination. Mutation occurs when part of the genetic material of the virus alters slightly. Some changes kill the virus or make it less potent. Other changes make it more deadly, more able to evade the host's immune system or more easily spread from host to host. Recombination occurs when two or more virus strains meet in the same host and trade parts of their genes. This can create new combinations of genetic material that sometimes bring together viral characteristics that were not in a single virus before.
Spread of Influenza
Because the virus is also mutating while it grows inside a person or animal, new variants that can better evade the immune response may form during an infection. This is why new strains appear every year and why infection with one strain does not guarantee protection against strains circulating in future years. Some strains are particularly virulent or deadly and these contribute to epidemics or pandemics, infections that spread around the globe.
