Avian diseases can cause chronic population declines, dramatic die-offs or reductions in the reproductive success and survival of individual birds. They can even cause extinctions. Certain avian diseases appear to be spreading to populations previously unaffected, including to species already threatened by other factors. Examples include:

1. Avian botulism, a bacterial disease that is arguably the most important disease of migratory birds world-wide, affects millions of birds. In 2002–2003, an outbreak in Taiwan, China killed more than 7% of the world population of Black-faced Spoonbill Platalea minor (Endangered) (Yu 2003).
2. West Nile Virus, a largely mosquito-borne viral disease (causing both bird and human mortalities), has established itself over much of eastern USA since 1999, spreading to Latin America and the Caribbean. American Crow Corvus brachyrhynchos has shown very high levels of mortality from this disease but remains relatively stable across its range (Ananthaswamy 2003, Bonter and Hochachka 2003).
3. Avian cholera and Erysipelothrix rhusiopathidae, two bacterial diseases, have caused considerable declines of Indian Yellow-nosed Albatross Thalassarche carteri (Endangered) on Amsterdam Island (French Southern Territories). The diseases may have spread to nearby colonies of Sooty Albatross Phoebetria fusca (Endangered) and Amsterdam Albatross Diomedea amsterdamensis (Critically Endangered with a world population of only c.130 birds) (Weimerskirch 2004). Avian cholera has also devastated the population of Cape Cormorant Phalacrocorax capensis (Near Threatened) in Western Cape Province, South Africa, killing c.13,000 individuals between May and October 2002 (Williams and Ward 2002).
4. Mycoplasmal conjunctivitis, an infectious disease, has recently caused a significant decline in the introduced population of House Finch Carpodacus mexicanus in eastern North America, and has started to spread to the native population of this species in western North America (Hartup et al. 2001).