On June 5, 1981, with characteristic scientific understatement, the federal Centers for Disease Control reported five "unusual" cases of a mysterious deadly disease afflicting young homosexual men in Los Angeles.

Five years to the day later, the "unusual" cluster has grown to an epidemic of more than 21,000 victims of acquired immune deficiency syndrome across the united States, more than half of whom have already died. And there is growing concern that more than 10 times as many Americans could have AIDS five years from now.

While it is difficult to predict the course of the epidemic, several AIDS experts said this week that given current trends, the cumulative number of U.S. AIDS cases could possibly rise to 250,000 over the next five years.

New estimates were prepared by the CDC for discussion yesterday at a private Public Health Service workshop of some 80 AIDS specialists who gathered in Berkeley Springs, W. Va., to draw up a new five-year government plan for dealing with AIDS.

A five-year projection of 250,000 cases by the end of 1991 -- with roughly 60,000 to 80,000 new cases that year -- is "in the ballpark of what we're considering," said one government official attending yesterday's meeting. "This is based in large part on the people we believe are already infected . . . The number one message is going to be to try to prevent more infections from occuring."

Government estimates suggest that about 1 million Americans may now be infected with the virus that can cause AIDS and that at least 15 to 20 percent of them may come down with the disease within five years after infection.

The best hope for such individuals would be the development of drugs to control the virus responsible for the slow destruction of the immune system that makes AIDS victims vulnerable to life-threatening infections and cancer. Should an AIDS vaccine be developed successfully in coming years, it would not help those infected but would prevent the disease in others.

In interviews this week, specialists in the field stressed that major scientific advances have been made since the disease was first noticed five years ago, but the disease is far more complicated and the epidemic far worse than was anticipated at the start.

Not only is there more widespread infection, but there has been growing recognition that the virus responsible for the disease may incubate in the body and not cause health problems for five years or more -- or perhaps a lifetime.

"In June of 1981, all of us realized that there was something very important happening . . . From the very beginning, it was a very unusual and very serious disease in young men. Few if any of us recognized the numbers of people that would ultimately be affected and the profound impact of the disease on our international society," said Dr. James Curran, head of the CDC's AIDS effort for the past five years.

"We've really learned a tremendous amount about this disease in the past five years, but a lot of what we've learned has not been reassuring. The mortality (R) is close to 100 percent, a very large number are already infected and inevitably many of them will become ill," said Curran's deputy, Dr. Harold Jaffe.

Although the first cases were in homosexual men -- who continue to account for three-fourths of the cases -- AIDS has spread through other segments of the population, largely through sexual contact or exposure to blood.

The disease has been recognized in intravenous drug abusers, Haitians, hemophiliacs, transfusion recipients who received blood before a screening test was in place, heterosexual partners of persons in high-risk groups, infants of infected mothers and some with no known risk factors for the disease. AIDS cases have been been reported in every state.

While AIDS experts took pride in the development of a blood test that has been used for more than a year to screen the nation's blood supply, they cautioned that research payoffs in terms of a safe and effective fective therapy or vaccine may be several years off.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said he thinks that it is realistic to assume that at the end of five years a vaccine would still be undergoing testing and that drugs to treat infected people will be "slightly to moderately effective" in suppressing the virus.

Although there may be a slow down in the growth rate for AIDS cases, he said AIDS will "clean emerge as a major killer of your people."

Dr. Walter Dowdle, head of the Public Health Service task force AIDS, said a key effort in the next five years will be education to risk groups as well as the general public.

"If vaccines and drugs aren't going to be available immediately for intervention, we'll rely heavily an information and education programs aimed at the groups at highest risk . . . The question is how we reach them," Dowdle said.

"Looked at from one perspective an incredible amount has happen over the past five years." says I Mervyn Silverman, former head of the San Francisco health department and president of the America Foundation for AIDS Research. But, he added, "all things being equal, the numbers could be through the roof."

Silverman said that, given to pool of individuals already infected "those numbers of 250,000 or more are very possible . . . Over the next five years, a lot is going to a pend on behavior," particular changes in sexual behavior by liting sexual partners and protected against exposure to bodily flue with condom, and other means.

There is general agreement to heterosexual spread will increase over the next five years, but do agreement about the rate. Despite widespread publicity about heterosexual cases, many experts at CDC and the National Institutes of Health said they think that the cases' spread will be slow and be creasingly significant but still anatively small minority of over cases in 1991.