Avian influenza is a flu virus that birds carry and infects them. The current strain of avian influenza circulating (H5N1) among birds has caused serious disease in a few humans that we know about, mostly in Asia. There has been human-to-human transmission in only two documented cases, and there was no further spread past one person. Therefore, it is not considered transmissible, but the potential exists for the flu virus to change and be easily transmitted from one human to another.
Most of the flu viruses that cause human disease are strains of avian influenza that have changed to become infectious to humans. Sometimes a human or a pig will become infected with an avian flu virus, and while in that person or animal, the virus undergoes changes that make it transmissible among humans. This is called reassortment, and it usually occurs when that human or animal is infected with both an avian flu virus and a human flu virus. The two types of viruses can then make a new virus that is then transmitted from human to human.