Epidemiologists have long suspected that genital herpes-herpes simplex type 2, as it is officially called-was very common, but until recently blood tests could not distinguish between the viruses that cause genital and oral herpes.

The racial gap may also be explained partially by the higher infection rate of younger blacks for herpes type 1, which causes mouth lesions, said Ward Cates, director of CDC's division of sexually transmitted diseases. People who have already had oral herpes generally have much less severe reactions to genital herpes, so they do not know they have the genital disease and do not take precautions against transmitting it, Cates said.

The high incidence of genital herpes is worrisome because the disease is associated with a high risk of AIDS, said Wendy Wertheimer, deputy executive director of the American Social Health Association. Not only does herpes often pinpoint people who have engaged in high-risk sexual activities, but genital lesions make one more vulnerable to infection from HIV, the virus that causes AIDS, she said.