West Nile virus.

For most of the twentieth century following its discovery in Uganda, West Nile virus, an organism that can cause severe inflammation of the spinal cord and brain, was known only in Africa, southeast Europe, and southwest Asia. As the century came to a close, however, the potentially fatal disease emerged in the Western Hemisphere, first in the New York City metropolitan area. Since then, it has spread across the continental United States, seven Canadian provinces, Mexico, the Caribbean, and parts of Central America. One of the hallmarks of the disease is the appearance of dead birds, such as crows, who are also susceptible to the virus.