What is Avian Flu?

Influenza viruses not only affect humans, but can affect other animal species like birds and pigs. Avian flu (bird flu) is caused by an influenza virus that circulates naturally among birds, usually without causing severe disease. 

Certain types of influenza viruses, the H5N1 subtype for example, are very pathogenic, or severe, and can kill infected birds.  Some viruses can not only kill the birds, but can be transmitted across species and infect humans.  When this happens, these viruses usually cause severe disease since people do not have immunity to them and there are no vaccines available for the disease.