There are many variations of influenza. However, there are three main types that are of interest around the world:
Annual human influenza
Human influenza viruses circulate every year. The viruses change slightly each year, so generally most people have some specific immunity. Scientists can generally track the variations to develop an annual vaccine to prevent influenza.Avian influenza
Bird flu affects birds and some mammals. In rare cases, this virus can spread from birds to people. Avian viruses are watched carefully in case they undergo a major change that could cause a human pandemic with significant illness and death among people.Pandemic human influenza
A pandemic would be the result of a major change in an influenza virus, for which people will have little or no specific immunity. A pandemic influenza could cause significant illness and death among people around the world. Although the severity is unknown, it would be widespread.