Bird flu has led to the deaths of hundreds of millions of wild and domestic birds and to a small number of human deaths. Right now, however, bird flu remains difficult for humans to contract. Most people who have developed symptoms have had close contact with sick birds, though in a few cases, bird flu has passed from one person to another.

Health officials are concerned that a major bird flu outbreak could occur in humans if the virus — H5N1 — mutates into a form that can spread more easily from person to person. The grimmest scenario would be a global outbreak to rival the flu pandemic of 1918 and 1919, which claimed millions of lives worldwide. For now, researchers are trying to develop a vaccine that would protect people in the event of a bird flu pandemic.