Avian influenza, commonly known as “avian flu” or “bird flu,” is caused by influenza type A viruses that normally only occur in birds. Avian flu is very contagious among birds and can make some domesticated birds, such as chickens, ducks and turkeys, very sick and kill them. These viruses usually do not infect humans, but in recent years several cases of avian flu infection in humans have been reported.
There are several subtypes of avian influenza A viruses. The subtype that has become of major concern is avian influenza A (H5N1) virus which has caused the deaths of millions of birds and also poses a health risk to humans.