West Nile encephalitis infection, carried by mosquitoes, can cause the brain to swell but rarely leads to death. Many people carry the virus with mild if any symptoms, but people with severe reactions may suffer convulsions, fever, and paralysis. There are no specific treatments for the disease. The virus had never been seen in North America until it killed seven people in the fall of 1999 in New York. The New York deaths occurred soon after birds began dying in New York City. Tests eventually identified West Nile encephalitis virus as the culprit. Birds form an odd relationship with mosquitoes, and viruses are the direct beneficiaries. In fact, when birds die en masse for no obvious reason, such as they did in New York in 1999, arboviruses can easily rank among suspected causes. Female mosquitoes bite animals for blood mainly to develop eggs. While feeding, they may ingest the virus from an infected bird and later may pass infection to another bird or other animal in their saliva when they bite again. Harvard researchers are trying to better understand such linkages in order to combat deadly virus outbreaks.