Epidemiology. The science of epidemiology is a broad framework on which a large variety of subjects can be developed. This diversity is clearly demonstrated in the seven epidemiology papers appearing this month. On page 127 is a report dealing with the prevalence of human T cell lymphotropic virus 1 and 2 antibodies in high risk populations in Tijuana, Mexico. Positive responses were seen in 21% of injecting drug users with lower numbers in prisoners, prostitutes, and homosexual/bisexual men. The authors conclude that there is a potential for rapid spread of human immunodeficiency virus 1 because of high-risk behavior and recommend binational cooperation for aggressive preventive measures. The potential impact of human immunodeficiency virus type-1 (HIV-1) infection on endemic tropical diseases such as Gambian sleeping sickness remains a matter for concern. Starting on page 133 are the results of a field study carried out in Zaire looking at cases of sleeping sickness and a control group of blood donors tested for HIV-1 antibodies.