NEW YORK -- The city of New York on Thursday launched an HIV-screening campaign that aims to test every adult in the Bronx, the city's borough most affected by AIDS, the health department said.

Forty locations in the region north of Manhattan, including hospitals, clinics, health centers and places of worship, are offering systematic HIV testing to all patients or visitors -- the largest HIV testing initiative in the city's history.

The objective is to test everyone in the Bronx ages 18 to 64 within the next three years. About 250,000 people, or three of 10 Bronx adult residents, have never been tested, the city estimates.

"We'd like to saturate the Bronx with testing," said Monica Sweeney, assistant commissioner for HIV prevention and control.

"Knowing your status is one of the best things you can do to stop the spread of HIV."

Friday is National HIV Testing Day in the United States.

Several community groups are partnering with the city to conduct the tests, mostly administered orally, which yield results in 10 to 20 minutes.

In 2006, 3,745 New Yorkers were newly listed as HIV-positive -- one quarter of them from the Bronx.

Manhattan, the city's most affluent borough, has more HIV-positive residents but fewer AIDS deaths than the Bronx, where a quarter of those infected only learn of their status once they have full-blown AIDS.

In a study released last week, New York's health department revealed that only 60 percent of New Yorkers who have had sex with more than one partner use condoms.