Public health officials, concerned about the growing number of young people exposed to the virus that causes AIDS, are stepping up efforts to screen adolescents and young adults for the disease in Washington, Baltimore and four other high-risk urban areas.

Some 100,000 people 21 years old and younger are believed to be infected with the human immunodeficiency virus (HIV), according to Project Access, a testing and counseling program that is sponsored by the federal Health Resources and Services Administration and the National Institutes of Health.

Of the 40,000 new cases of HIV diagnosed annually in the United States, "about 50 percent occur in people less than 25 years of age, and about 25 percent are in people 21 and younger," said Helene Gayle, director of the National Center for HIV, Sexually Transmitted Diseases and Tuberculosis Prevention. Two young Americans are infected with HIV every hour, according to the Centers for Disease Control and Prevention (CDC).

Because symptoms of AIDS take about 10 years to develop, estimates are that a third to a half of HIV-infected people don't know that they have the virus. "Among teens, it is more likely to be half who don't know it," Gayle said. "They're still healthy and asymptomatic and so would not be seeking testing."

Adolescents are among the least likely of all age groups to have regular contact with the medical profession. Without getting tested to detect the virus, many infected teens go undiagnosed.

That means no access to life-prolonging medical treatment that can help delay the symptoms of full-blown AIDS. It also means that those infected can't take steps to help protect their sexual partners.

"Health providers that do come into contact with teens are not fully aware of this HIV epidemic in teenagers and don't routinely offer testing," said Donna Futterman, chair of Project Access and director of adolescent medicine at Montefiore Medical Center in New York, where Project Access began. Besides New York, Washington and Baltimore, it also now operates in Miami, Philadelphia and Los Angeles.

Adding to the problem is that most HIV programs don't target teens. They don't warn this age group of the importance of testing, or emphasize that having unprotected sex is a risk factor since the virus can be transmitted during sex. "Unless we speak to teens in their language and [in] ways directed to them, they are not going to believe that HIV has anything to do with them," Futterman said.

To help educate teens about HIV, Project Access will offer free testing and counseling for people ages 13 to 24 from Dec. 1 through Dec. 7 in the District, Baltimore and the surrounding metropolitan region.

The testing takes only minutes and is performed on saliva, rather than by drawing blood. The saliva test measures the presence of antibodies or proteins formed against HIV, not levels of the virus itself. Results are available within a week. Immediate counseling is provided for those who are HIV-positive and all infected teens and young adults are referred to nearby medical facilities for treatment.

Washington has one of the the highest rates of HIV infection in the nation, according to Project Access. From 1996 to 1997, AIDS cases among adolescent females in the District rose by 60 percent and among male teens by 45 percent. Among states Maryland has the third highest rate of HIV infection in adolescents, while Virginia, ranks fifth for HIV-infected males aged 16 to 21 years.

Where once the HIV epidemic was largely confined to homosexuals and those using intravenous drugs, the infection is increasingly diagnosed in young women, who account for about a third of the newly diagnosed cases, according to the CDC. In some areas, including Washington, the prevalence appears to be even higher. At Children's National Medical Center in the District, nearly 70 percent of the new cases diagnosed occur among young women, who have acquired the disease almost exclusively from heterosexual contact.

Sixty percent of the HIV-infected patients at Children's have been diagnosed with another sexually transmitted disease or have been pregnant before, said Lawrence D'Angelo, medical director of the HIV/AIDS clinic at Children's, which is coordinating the Washington efforts of Project Access. This indicates that many young people are engaging in a chronic pattern of unsafe sex, he said.

Health officials are concerned that teens who know they are infected often remain sexually active, placing others at risk. "Teens with HIV infection inform their 'main sexual partner' of their diagnosis 50 percent of the time and 'casual' partners only 15 percent of the time," D'Angelo said.

Those same trends are also emerging throughout the nation. "We assumed kind of naively that you could do prevention for one generation and that it will trickle down to the next," said the CDC's Gayle. "It doesn't."

While many of the public health messages have successfully slowed the AIDS epidemic among white older gay men, the rates of HIV infection among the younger generation of gay men in the African American and Hispanic communities are rising, just as they are among young women.

"We need to maintain good prevention messages and services," Gayle said. "We need testing and counseling. We're starting to see a rise in other sexually transmitted diseases because people are returning to or initiating risky sexual behavior."

WHERE TO GET TESTED:

The Project Access counseling and free testing program for people ages 13 to 24 will run Dec. 1-7 in Washington, Baltimore, Philadelphia, New York, Miami and Los Angeles. For information on locations and hours in the Washington area, call 202-547-8719. For information about sites in the Baltimore area, call 410-706-6000.

Project Access partners in Washington and surrounding area are:

* Alexandria City Health Department, 517 N. St. Asaph St., Alexandria. 703-838-4400.

* Arlington County Health Department, Department of Human Services, Communicable Diseases Unit, 1800 North Edison St., Arlington. 703-228-5176.

* Family & Medical Counseling Service Inc., 2041 Martin Luther King Jr. Ave. SE. 202-889-7900.

* Healthy Teens Center, Prince George's County Health Department, 7824 Central Ave., Landover. 301-324-5141.

* Metro TeenAIDS, The Stop, 2412 S. Shirlington Rd, Arlington. 703-769-STOP.

* Metro TeenAIDS, The House, 3311 Toledo Terrace, Hyattsville. 301-270-7337.

* Metro TeenAIDS, Free Style, 1523 Pennsylvania Ave. SE. 202-547-8799.

* Planned Parenthood, Ophelia Egypt Center, 3937-A Minnesota Ave. NE. 202-388-4770.

* Planned Parenthood, Hyattsville Center, 1835 University Plaza, Hyattsville. 301-422-6066.

* Prince George's County Health Department, HIV/AIDS Prevention Department, Penn Silver Health Center, 508 Silver Hill Rd., Forestville. 301-817-3180.

* Whitman Walker Clinic, Elizabeth Taylor Medical Center, 1701 14th St. NW. 202-745-7000.

* Max Robinson Center, 2301 Martin Luther King Jr. Ave. SE. 202-678-8877.

* Washington Free Clinic, 1525 Newton Street NW. 202-667-1106.