Influenza describes the illnesses caused by the orthomyxoviridae family of viruses. These viruses are generally spread via respiratory droplets and can cause a variety of symptoms, including a fever, muscle aches, chills and a cough due to their ability to infect the respiratory system. There are many different strains of flu viruses, but in general the flu can be divided into types A, B and C viruses.
Influenza A
Influenza A viruses are able to infect birds, pigs, humans, horses and other animals. Wild birds, however, are the natural hosts for these viruses. There are many different subtypes of influenza A viruses, which are distinguished from each other based on two different proteins, known as hemagglutinin and neuroaminidase, often abbreviated "H" and "A" respectively. The hemagglutinin protein is needed for the virus to infect host cells and the neuroaminidase protein works to help make new copies of the virus once an infection has occurred.
There are 16 different types of hemagglutinins that have been identified and nine different neuroaminidase proteins, the U.S. Centers for Disease Control and Prevention explains. Different subtypes of influenza A are typically named after the type of hemagglutinin and neuroaminidase protein that the virus uses. The swine flu virus was also named H1N1, for this reasons, whereas the avian flu virus is also known as H3N2.
Influenza viruses are able to change their genetic makeup via a process known as antigenic shift and drift; Influenza A is the only variety that can perform antigenic shift. Antigenic shift refers to major changes in the genetic makeup of the virus due to the sharing of genes between different subtypes of Influenza A. When this happens, the virus can undergo dramatic changes which can result in a pandemic, as human immune system's will not be equipped to combat this new form of the virus. For this reason, Influenza A is most commonly associated with serious pandemics and more serious cases of the flu, such as the H1N1 virus which recently came into prominence.
Influenza B
Influenza B causes similar symptoms as Influenza A, the Center For Disease Control explains. This class of viruses is not broken down into multiple subtypes, however, and it generally does not cause as severe a disease as Influenza A. This may be the result of the inability of the Influenza B virus to undergo genetic shift. Influenza B is still able to change its genetic material through a process known as genetic drift, in which small changes in the genes of the virus produce minor modifications to the viral structure. This can make it difficult for the immune system to recognize the new forms of the virus, which prevents long-term immunity. However, because this process produce much smaller changes in the DNA of the virus compared to genetic shift, the effects of genetic drift are less dangerous. There are two different types of Influenza B virus, according to an article published by FierceVaccines.com, although most influenza vaccines only contain a vaccine for one of the different Influenza B types.
Influenza C
Influenza C is a very common viral infection, according to Dr. Racaniello at Columbia University. Most adults have been infected with Influenza C at some point in their life, but the virus usually only results in a mild infection of the upper respiratory tract. Consequently, there is no vaccine for Influenza C.
