Nearly three years after the first A.I.D.S. cases were detected in the United States, a lot of countries throughout the world have reported varied numbers of such cases. While several U.S. towns have to face up to an epidemic situation, the same phenomenon has not been observed yet in the other areas, including European countries. In those latter countries homosexuals represent the most important group among patients. Two differences have to be noticed with the U.S. situation: numerous cases among patients from Equatorial Africa have been reported, and the disease has not spread over among I.V. drug abusers. Although no specific agent can be identified yet, A.I.D.S. features are characteristic of a transmissible disease.