There are three types of influenza viruses: A, B, and C. Of these, type A is further classified into subtypes, H and N. Type A affects people and numerous animals, while B and C typically affect humans only. Each type varies in severity, with A being the most dangerous, then B, and C being the least severe.

Bird flu is of the A type of influenza virus, one of more severe types. While several subtypes and strains exist, H5N1 is the most widely known and feared form of bird flu.

One of the primary differences of bird flu and the other types of the influenza virus is the way it is transmitted. Other strains of the flu virus, common among humans, are passed from human to human through saliva and mucus. This is done through sneezing, coughing, or breathing on or near someone. H5N1, on the other hand, is only transmitted from poultry to humans, and is done so through direct contact with the animals or their feces.

Another difference between the regular flu virus and H5N1 is the availability of a vaccine. The regular flu has been an irritant to people for some time. As a result, steps were taken long ago to challenge the virus and lessen the impact it had on people. Every year, people go in droves to receive a flu vaccine to avoid the virus. However, a vaccine has not been developed for the mutated form of bird flu that is to come. The reason for this is multifaceted. One reason is the lengthy development process. H5N1 has been a serious threat to humans for less than two years, since the outbreak of the virus began in Vietnam in January of 2004. Therefore, there has not been significant time to research and create a vaccine for the virus. Also, little purpose lies in creating a vaccine for the current strain of H5N1, as the real threat will come when the virus mutates. A vaccine that is developed now will be useless when mutation occurs, at which point a vaccine will really be needed. So, the virus must first be allowed to mutate and only then can an effective vaccine be developed.

The other variation between bird flu and other flu strains is the severity of the symptoms. While symptoms of sneezing, coughing, sore throat and fever are similar, the symptoms easily become severe in cases of bird flu.