Ducks and avian influenza virus particles, composite image. The virus particles (brown) have been imaged using a transmission electron microscope (TEM). Avian influenza (bird flu) is a disease of wild birds, which are usually resistant, that can spread to domestic birds, for whom it is almost invariably fatal. Humans can contract the disease through close contact with birds and their droppings. A new strain, H5N1, was found in Southeast Asia in the late 1990s, and has since killed over 100 people. There are fears it may mutate to a form capable of spreading from human to human, which could cause a devastating pandemic.