Federal health officials say they are concerned that exposure to the AIDS virus is far more widespread than earlier suspected, and that at least 300,000 people already may have been unknowingly infected.

The future of this group, most of them homosexual men, is uncertain. But Dr. James Curran, head of the AIDS task force at the Centers for Disease Control in Atlanta, said preliminary studies suggest that perhaps 10 percent may come down with the severe form of the deadly acquired immune deficiency syndrome, and twice as many may become sick with milder symptoms.

"There's a tendency for all of us to underestimate the problem because we don't want to believe it's true," Curran said.

Curran said that the more than 6,600 U.S. victims of AIDS, nearly half of whom have died, may be only a hint of a wider problem.

The greatest risk is still among sexually active homosexual men, intravenous drug users, hemophiliacs, Haitians, and their sexual partners. Officials acknowledge, however, that the problem is likely to worsen until researchers can translate recent dramatic scientific progress into effective treatment and prevention.

CDC's pessimistic new estimates about the magnitude of the AIDS problem are a result of this progress. Identification last spring of a virus that is widely accepted as the cause of AIDS led to a blood test that is revolutionizing scientists' ability to study the disease.

But like many new technologies, the AIDS test is double-edged. In the long run, it will allow scientists to unravel the mysteries of AIDS. But until then, while offering hope for some, it brings potentially alarming news, but no immediate help, to others.

In addition to its growing use in research studies, a commercial version of the test already is being used on humans. Five companies under government license are racing to develop a large-scale test by early next year to screen the nation's blood supply for evidence of AIDS contamination.

This would provide protection for those receiving blood transfusions. But there is concern about the impact of the test on individuals who show evidence of infection.

The test is a screening tool that reveals only that a person has been exposed to the AIDS virus, but does not tell whether the virus is active.

AIDS appears to be caused by a virus -- one of a group called retroviruses -- that can enter the body and perhaps stay for a lifetime, unlike most viruses, which cause a disease shortly after exposure, or may produce immunity. Scientists still do not know enough about the disease to predict whether a person exposed to the AIDS virus will develop the disease.

But the data so far is "very disturbing," said Dr. Harold Jaffe of the CDC. He said recent blood-test studies suggest that exposure to the virus is quite high among certain groups, that the average incubation period between exposure and development of the disease is approaching three years, and that the chance of becoming ill appears to be higher than expected.

A follow-up study of 6,800 homosexual men attending venereal disease clinics in San Francisco found that two-thirds of the men had been infected with the virus. Of those who had been infected several years before, one-fourth developed symptoms of an illness that may sometimes precede AIDS, including swelling of the lymph nodes, extreme fatigue and weight loss.

Another 7 percent developed AIDS, in which viral destruction of crucial white blood cells makes the victim more vulnerable to life-threatening infections and cancer.

Recent studies at the National Cancer Institute showed similiar trends, said Dr. William Blattner. But he cautioned that the disease pattern in homosexual men may not mirror other groups, and that more follow-up is needed.

Curran said he believes, however, that there is enough evidence to suggest that the number of Americans exposed to AIDS is already "30 to 50 times as large as the number of cases that have been reported to date. It is likely a substantial percent of those exposed , from 5 to 20, will come down with AIDS. The best estimate is 10 percent" over the next five years.

Dr. Edward N. Brandt Jr., assistant secretary for health in the Department of Health and Human Services, who targeted AIDS as the nation's No. 1 public health problem, is more cautious. He said he considers such projections "unnecessarily alarmist . . . . I would not be willing to make any predictions."

But Dr. Robert Gallo, the National Cancer Institute scientist who discovered the AIDS virus -- Human T-Cell Leukemia Virus III or HTLV-III -- said he considers Curran's estimates conservative.

"I think it is far more than a few hundred thousand who have been exposed . . . ," Gallo said. "There is a fear of not causing panic. But if you get too cautious, you underestimate a problem that may be growing."

The government hopes to get a better grip on the problem with several National Institutes of Health studies.

One study, sponsored by the National Heart, Lung and Blood Institute, is gathering 200,000 blood samples from Los Angeles, San Francisco, New York, Miami, Detroit and Seattle. It seeks to quantify the relationshp between a positive AIDS blood test and later development of AIDS symptoms.

Four of the five companies developing diagnostic tests already are conducting tests on human blood in about 30 centers around the country. Dr. Lowell T. Harmison, a Public Health Service official who is coordinating the effort, said that if all goes well, the test could be approved early next year.

Both Harmison and Brandt said the initial group of tests will go for research, but they hope it soon will be implemented routinely to screen and discard all donated blood with signs of virus infection. Individuals in high-risk groups already have been urged not to give blood.

A key issue with the new blood test is its accuracy. Of particular concern is separating true positive results from "false positives."

In the laboratory, the test has shown an initial "false-positive" rate ranging from less than one-half of 1 percent to 2 percent. Brandt, Harmison and Gallo said they believe that the commercial test will prove to be highly specific, but others worry privately that even a small, false-positive rate could pose a problem after an estimated 20 million tests are conducted annually.

If a test is positive, most officials agree that the result should be given to the blood donor, despite the great uncertainty in saying what it may mean for the individual's health. "It's important to know about it in terms of personal health," said Harmison, not only in terms of seeking medical follow-up in the future but in changing personal habits to prevent the chance of spreading the disease to others.

AIDS does not appear to be spread casually, but only through intimate contact with body fluids, particularly through sexual contact, or through exposure to blood.

The diagnostic test has drawn mixed reaction in the homosexual community. Jeffrey Levi of the National Gay Task Force said that the test is important for research and blood screening, but he expressed concern that the test, in the wrong hands, could be "used as a surrogate marker for homosexuality . . . . It could be used by insurance companies or employers to discriminate."

Brandt acknowledged the fears, but said "public health concerns about the blood supply override some of the other aspects . . . . It's still undergoing a lot of active debate."

He and others said, however, that transmission of AIDS through the blood supply is only a small part of the problem. The only real hope for controlling the epidemic will be through development of anti-viral drugs and, possibly, a vaccine.

"The epidemic won't be stopped dead in its tracks for another three to five years," Brandt said. Even if progress is made, others emphasize that a disease such as AIDS likely will never be eradicated.