Killer Flu Viruses
Learn more about influenza viruses

The Three Types of Influenza Viruses:
Influenza A, B, and C
Influenza types A and B viruses are the most common, causing epidemics of disease almost every winter. In the U.S., these winter flu epidemics cause illness in up to twenty percent of the population and are associated with an average of 114,000 hospitalizations and 36,000 deaths per year. You can prevent illnesses from types A and B influenza by getting a flu shot. Type C influenza infections do not cause epidemics, although they do cause respiratory illnesses. The flu shot does not protect against type C influenza.
Type A influenza viruses are further divided into subtypes that are determined by two proteins on the surface of the virus. These proteins are called neuraminidase and hemagglutinin. Subtypes of influenza type A viruses found in people are called A(H1N1) and A(h3N2). Influenza B virus is not divided into subtypes. These different strains are included in each year’s influenza vaccine.
Influenza virus transmission between people and animals
Influenza A viruses can be found in different animals, including whales, horses, seals, pigs, chickens, and ducks. Influenza B viruses, on the other hand, only circulate widely among humans.
As stated before, influenza type A viruses contain two proteins that determine their subtype—neuraminidase (N for short) and hemagglutinin (H). There are sixteen different hemagglutinin subtypes and nine different neuraminidase sybtypes. These have been found in influenza A viruses in wild verbs.
In fact, wild birds are thought to be the primary reservoir for all subtypes of influenza A viruses and are thought to be the source of influenza viruses in all other animals.
While most influenza viruses cause asymptomatic or mild infections within birds, the range of symptoms varies wildly depending on the virus strain. Infections with certain avian influenza A viruses can cause widespread death and disease among wild and domestic birds. Chickens and turkeys are especially prone to these diseases.
Pigs can also be infected with both avian and human influenza viruses in addition to its own swine viruses. Infected pigs get symptoms similar to that of humans including fever, cough, and runny nose. As pigs are susceptible to human, swine, and avian viruses, they might become affected from influenza from different species at the same time. If this were to occur, it is possible for the genes of these viruses to mix and create a new virus.
For instance, if a pig were to be infected with a human virus and an avian virus at the same time, the virus could mix and produce a new virus that had most of the genes from the human virus and an additional protain (H or N) from the avian virus. This new virus would then have the potential to spread and infect humans. This mutation in viruses is known as an antigenic shift. These shifts happen when a new influenza A subtype infects humans with little or no immune protection. Fatal influenza epidemics occur in this way.