With a jangle of bracelets, Patricia Shelton slid her chair in front of the air-conditioner in her daughter's apartment and fluttered her hands to cool her face.

"I swear, some days it's the menopause that gets me, not the H.I.V.," she said.

At 51, what she calls "being H.I.V." never really gets her. She has known she was infected since 1990, "the same time Magic Johnson announced to the world."

She is still on the two-drug regimen she started on, and her viral load is too low to be detected. But she leads workshops for older infected adults, and "I know I am very blessed," she said. "Some of them are on their fourth regimens, get bouts of PCP pneumonia, rashes, herpes, diarrhea."

In her 20's and 30's, she was a "closet heroin addict," keeping a Wall Street secretarial job, raising her children, not losing control. "A lot of us who had a past are happy housewives now, are mothers and grandmothers, are productive members of society," she said.

The infection lingers, but she has proved wrong the doctor who told her in 1990 that she had two years to live.

Although AIDS is thought of as a disease of the young, in the United States it is rapidly becoming one of the middle-aged and even the old. The number of Americans over age 50 infected with the virus that causes AIDS quintupled during the 1990's, "and a conservative estimate would be that there are more than 100,000 now," said Dr. Marcia G. Ory, a professor of public health at Texas A & M University and co-author of a 2003 report for the Centers for Disease Control and Prevention on AIDS in older Americans. Unless there is a new explosion of the disease among teenagers, demographers estimate, the majority of cases by the end of the decade will be in people over 50.

In New York City, the curve has moved even further. About 64 percent of the city's cases are over 40 right now, the New York City Department of Health said, and about 25 percent are over 50.

The medical and social ramifications of this shift are already becoming evident, particularly as the cost of care escalates.

"There will be some reality checking very soon," said Dr. Stephen Karpiak, research director at the AIDS Community Research Initiative of America, or Acria, a nonprofit group based in New York that does surveys and clinical trials. "People are already being assigned to nursing homes at age 55. That gets very expensive."

In large part, the changing demographic of the disease is a testament to medical progress. Thanks to a growing armory of antiretroviral drugs and advances in the way secondary infections are fought, the infected live longer. Many have heard from their doctors words that are strangely gratifying: You're getting old, and you'll die of something, but it won't be AIDS.

The increase is also, in part, statistical. Very few newborns now get the virus from their mothers, and very few hemophiliac children get it from blood products, so the average age of the infected has climbed. But there is a countervailing pressure; blood transfusions were once a major cause of AIDS among those over 50, and that risk has all but vanished.

There is also a new pool of cases, those who contract the infection later in life. In a 1999 C.D.C. survey, 44 percent of infected people aged 60 or over did not know how they encountered the virus. Only 30 percent of those under 50 did not.

Dr. Karpiak's team has interviewed 160 infected people over the age of 50 and plans to interview 1,000 more to assess the challenges of treating older patients. The preliminary results uncovered some problems.

For example, 71 percent lived alone. "That really struck me," Dr. Karpiak said. "That's the antithesis of the regular population, where 30 percent live alone."

More than half said they were not dating. Although most had living children, siblings or parents, only 23 percent said they looked to them first for emotional support or for help with chores like going to the store or changing a light bulb. More asked friends, and 26 percent said they relied on themselves or no one.

In Dr. Karpiak's survey, 79 percent said they needed more help with daily tasks like cooking, cleaning and transportation. Depression, inability to get out and forgetfulness about pill-taking may speed their declines.

Gay elderly people often have no children, and former addicts may be estranged from their families. In both groups, many may have already buried most of their old friends.