Abstract in English:Highly active antiretroviral therapy (HAART), which has been provided since 1995 to patients infected with human immunodeficiency virus (HIV), has produced noticeable results in regard to preventing opportunistic infections in this population. Nevertheless, it is still useful to administer specific prophylactic treatment against opportunistic infections in patients treated with HAART and also in patients who do not respond to HAART or who cannot or do not wish to take that treatment. Prophylactic treatment can be halted when, due to HAART, there is a sustained increase in the CD4+ T-lymphocytes count (that count indicates the degree of susceptibility that a person has to opportunistic infections). Guidelines for preventing opportunistic infections were developed in 1995 by the United States Public Health Service and the Infectious Diseases Society of America, and they have been reviewed periodically. The last review, done in 1999, marked the introduction of important changes regarding clinical parameters to halt primary and secondary prophylaxis against Pneumocystis carinii pneumonia, Toxoplasma gondii, and Mycobacterium avium complex. These changes, along with the latest information concerning the transmission of human herpesviruses, immunization of children infected with HIV or exposed to the virus, and certain drug interactions, are the major focus of this review.