Type A virus can affect animals, but most commonly affects people. This virus is typically responsible for the larger flu epidemics and is spread by people who are already infected. According to the CDC, "The emergence of a new and very different influenza A virus to infect people can cause an influenza pandemic." Since the flu strain is new, people don't have immunity to it so it can spread quickly.
Influenza A viruses are divided into subtypes determined by the two proteins found on the surface of the virus: hemagglutinin (H) and the neuraminidase (N). Currently, there are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes.
The Type A flu virus can be further broken down into strains. The H1N1 and H3N2 strains are found in people, and in 2009, the CDC found that a new H1N1 virus (also known as 2009 H1N1) causes illness in people and caused the first flu pandemic in 40 years.