Influenza Viruses (Flu Viruses)
Three types of influenza viruses affect people, called Type A, Type B, and Type C. Usually, the virus is spread through the air from coughs or sneezes. It can also be spread by touching surfaces contaminated by the virus and then touching the mouth or eyes.
See also: About Microbes, Protecting Yourself From Respiratory Infections
About Flu Viruses
Influenza, or flu, is a respiratory infection caused by any of several flu viruses, which are classified as type A, B, or C based on their nucleic acids and protein composition. Type A viruses are found in humans and many animals, including ducks, chickens, pigs, and whales.
Type A influenza viruses caused the global flu outbreaks of 1918, 1957, 1968, and 2009. Type B viruses circulate widely in humans. Type C viruses are found in humans, pigs, and dogs and cause mild respiratory infections but do not spark epidemics.
The outer coat of the virus features two kinds of protein spikes: hemagglutinin (HA), which allows the virus to stick to a human or animal cell and initiate infection; and neuraminidase (NA), which enables newly formed viruses to exit the host cell.
There are 16 HA subtypes and 9 NA subtypes among Type A viruses. Strains are named based on their combination of HA and NA proteins; for example, the virus that caused the 2009 H1N1 flu pandemic contained H1 and N1 surface proteins.
Inside a flu virus are eight segments of single-stranded RNA containing the genetic instructions for making new copies of the virus. NIH – National Institute of Allergy and Infectious Diseases