Bird flu, or avian flu, is a variation of the flu virus carried by birds. It is believed that birds pass the flu to domestic poultry sources such as chickens. These fowl pass the flu on to humans. Bird flu usually does not spread far from human to human. Symptoms are similar to those of the regular flu virus. The symptoms can progress to include respiratory disease, brain problems, and digestive problems. There is a vaccine, but it is not open to the general public yet. Follow the links below to find WebMD's comprehensive coverage about how bird flu is contracted, what it looks like, how to treat it, and much more.