There are at least 15 different types of avian influenza that routinely infect birds around the world. The current outbreak is caused by a strain known as H5N1, which is highly contagious among birds and rapidly fatal. Unlike many other strains of avian influenza, it can be transmitted to humans, causing severe illness and death.
Bird flu is not the same as SARS (severe acute respiratory syndrome). Although their symptoms are similar, SARS is caused by completely different viruses. Influenza viruses also are more contagious and cannot be as readily contained as SARS by isolating people who have the infection.
Influenza viruses are highly unstable and have the ability to mutate rapidly, potentially jumping from one animal species to another. Scientists fear the bird flu virus could evolve into a form that is easily spread between people, resulting in an extremely contagious and lethal disease. This could happen if someone already infected with the human flu virus catches the bird flu. The two viruses could recombine inside the victimâ€™s body, producing a hybrid that could readily spread from person to person.
The resulting virus likely would be something humans have never been exposed to before. With no immune defenses, the infection could cause devastating illness, such as occurred in the 1918-19 Spanish flu pandemic, which killed an estimated 40 million to 50 million worldwide.