Half of the Americans recently diagnosed with HIV had been living with the virus for at least three years without realizing it, missing out on opportunities for early treatment and in some cases spreading it to others, according to a new report by the U.S. Centers for Disease Control and Prevention.

The human immunodeficiency virus, or HIV, is responsible for causing AIDS. Infection was considered a death sentence until antiretroviral medications capable of suppressing the virus came into broad use in the late 1990s. But people can’t get the drugs they need if they don’t know their HIV status, said Dr. Patrick Chaulk, assistant commissioner of the bureau of HIV/STD services at the Baltimore City Health Department.

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