infections
Influenza (Flu)
Influenza types and strains
There are three types of influenza virus: Type A Type B Type C There are many different strains within types A and B influenza viruses but all cause similar illness. Different strains cause the regular seasonal outbreaks of flu. Type C does not cause a typical influenza illness. Type A causes most influenza cases (usually more than 70% in a typical season), and most of the others are caused by type B. Type C influenza occurs less frequently, mainly in children. The strain of influenza virus causing outbreaks is always changing slightly, so that each year the influenza virus is a little different from the previous year's. It often changes enough that previously effective vaccines no longer work. Influenza type A strains are named based on the specific versions of two proteins present on the surface of the virus. The proteins are H (for hemagglutinin) and N (for neuraminidase). There are 18 different H proteins and 11 N proteins. Thus, a strain might be named influenza A, H1N1. An H1N1 strain was responsible for the so-called swine flu pandemic in 2009–2010. (A pandemic is a major worldwide epidemic.) More recently, H3N2 strains have been causing most infections in the United States. The name of a strain often reflects its type, the location where it first appeared (for example, Hong Kong flu) or an animal (for example, swine flu), and the year it was detected. 