Influenza (commonly called the flu) is a highly contagious illness that can occur in children or adults of any age. This illness is caused by the influenza viruses that infect the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and at times can lead to death. The best way to prevent the flu is by getting a flu vaccine each year.
There are three different types of flu viruses: influenza A, B, and C, all of which cause illness in humans. Influenza is a virus that actually has hundreds of different strains. The virus mutates frequently, but the strains are classified into one of three main categories—A, B, or C. Influenza A is the group that most commonly causes illness in humans.
All Influenza A viruses are further broken down into H and N subtypes. So, any influenza virus that is described as “H#N#” (such as H1N1) is an influenza A virus. There are 16 H subtypes and nine N subtypes, but only three combinations have actually caused highly contagious illness in humans. The three combinations that cause almost all outbreaks of the flu in humans are H1N1, H2N2 and H3N2.