HIV infects 1 in 100 in New York.

A recent change in how New York City officials identify and track cases of HIV infection has yielded the dearest picture yet of how deeply rooted that city's epidemic has become. More than 75,000 currently living New Yorkers, or about 1 percent of all residents, have been diagnosed with HIV, says Denis Nash of the New York Academy of Medicine. Past studies suggest that undiagnosed infections could swell that number to more than 100,000.

Certain groups have a particularly high rate of HIV infection, Nash says. For example, 3.9 percent of 40-to-49-year-old men throughout the city have tested HIV positive, as have 2.8 percent of all men living in Manhattan. Nearly two-thirds of infections diagnosed in 2001 were in men, but the proportion of new diagnoses in women appears to be increasing.

The new data are the first to come from official alerts, compulsory since mid-2000, that HIV-testing laboratories and health care workers make to the New York City Department of Health and Mental Hygiene. Nash analyzed the data with colleagues while working there. Previously, labs and doctors systematically reported only diagnoses of AIDS, which can take years to develop after HIV infection.--B.H.