Casals's team at Yale University found the Lassa virus in the blood of three American missionary nurses who had contracted Lassa fever in northern Nigeria. The Yale team obtained evidence of a new virus and, following custom, named the virus for the place where it was first detected -- a village in Nigeria named Lassa.
The discovery of the virus and the laboratory accidents surrounding it -- a lab colleague of Casals also contracted Lassa fever -- led to new standards of laboratory safety. The Lassa virus story also led to recommendations for stricter isolation precautions in transporting sick passengers.
Lassa virus was not the only virus that Dr. Casals discovered. He found many. He also developed an effective classification system for viruses, particularly those spread by mosquitoes and other insects.