Influenza A Virus
One of three types of virus that cause the illness called influenza (flu). The influenza A virus can infect people, birds, pigs, horses, and other animals. It is the main cause of most influenza epidemics.
See also: Protecting Yourself From Respiratory Infections, About Flu Vaccines
About the Influenza A Virus
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. NIH – National Institute of Allergy and Infectious Diseases