WebMD defines type A flu as a type of influenza that causes coughing, body aches, high fever and sniffling. This virus usually infects humans, but it is also capable of infecting animals. Wild birds are often identified as the hosts for influenza A.
The CDC says influenza A is one of the viruses that causes annual flu epidemics. The virus is classified according to the two proteins found on its surface. These proteins are called neuraminidase, which has 11 different subtypes, and hemagglutinin, which has 18 different subtypes. The subtypes are used to indicate which type of influenza a person has. For example, the notation "influenza A (H1N1)" indicates infection with an influenza A virus that has the H1 and N1 protein subtypes.
In some cases, influenza A causes a flu pandemic, which is a global outbreak of a particular disease. The disease spreads quickly because humans have very little immunity to viruses to which they have never been exposed. A pandemic is very serious because even healthy people may experience serious complications. The Flu.gov website reports that pandemic flu may cause more severe symptoms and kill more people than seasonal flu. In 2009, H1N1, which is often referred to as swine flu, was responsible for the deaths of more than 18,000 people, according to NPR.