In late 1980, a 4-month-old baby entered a Newark, N.J., hospital with pneumonia. Within a month, she was dead, her immune system devastated.

More than a year later a heroin addict was referred to a Newark laboratory for blood tests. He was a suspected victim of Acquired Immune Deficiency Syndrome (AIDS), a new disease that wipes out the body's immune defenses and leaves its victims vulnerable to cancer, pneumonia and a host of other deadly infections.

Dr. James Oleske recognized the addict. He was the infant's father.

"That was a dramatic moment," Oleske said. "A light was turned on. I was convinced at that point there was a possible connection."

The death of a baby, and the chance discovery that her father may carry AIDS, is one of the clues scientists are using to track the elusive cause of the new American epidemic. Despite one of the longest and most extensive searches in the history of the Public Health Service, doctors don't know what causes AIDS or exactly how it spreads.

It was first discovered in 1981 in the homosexual communities of New York and Los Angeles. Then it turned up in drug addicts, Haitians, children and sexual partners of AIDS victims. Some hemophiliacs and recipients of blood transfusions also have begun to die of AIDS.

Whatever it is, modern life seems to nourish it, and life styles and technology seem to spread it.

One researcher described it as torture. "With other diseases you die very fast. This is just horrible, a dwindling of the healthy body. I've worked with miserable diseases, but none as miserable as this."

As of March 9, 1,145 Americans had contracted AIDS; 428 of them are dead. Twenty percent of the cases appeared in the last two months. Half of the victims are under 35.

It may be one of the most lethal diseases of modern times, more deadly than smallpox once was. So far the overall death rate among AIDS victims stands at 40 percent, but it gets worse as time goes on. More than 70 percent of those diagnosed in 1981 have died. None has recovered.

Modern medicine can treat only symptoms and complications--at a cost of about $200,000 per person. But the body's immune system cannot be repaired.

The disease once was called the gay plague. When hemophiliacs began to get it last year, concern arose that the nation's blood supply might be threatened. Finally, a baby boy showed signs of AIDS after receiving massive blood transfusions for Rh disease, a blood incompatability with his mother.

One of the baby's donors, it turned out, died of AIDS eight months after giving blood. The baby died a few weeks ago as he neared his second birthday.

Early this month, the Public Health Service asked drug users, Haitians and sexually active homosexual men to refrain from giving blood until AIDS can be stopped.

Representatives of the homosexual community complained bitterly that victims were being blamed, but the American Red Cross agreed to implement the new policy.

Rep. Henry A. Waxman (D-Calif.), chairman of the House Energy and Commerce subcommittee on health and the environment, said he believes that the public response to AIDS has been cautious because the disease appeared first among homosexual men.

"I don't think the Reagan administration could have asked for money fast enough if the disease had broken out instead among the American Legion or the Chamber of Commerce," he said. Congress intervened to increase funding for AIDS this year.

"It's very frightening," said Oleske, the Newark immunologist. "There's nothing more tragic . . . . Part of my frustration is the real lack of support for this among the powers that be" in Washington.

Dr. William Foege, head of the Centers for Disease Control in Atlanta, says, "AIDS is unique. It's so devastating." His agency has stretched its resources to study AIDS. "At the moment, not only is the incidence increasing but groups at risk seem to be enlarging over time . . . . It's a scary thing."

In January, 1981, a New York homosexual developed a rare form of cancer, followed by hepatitis and two serious bouts last year of an unusual type of pneumonia. He moved to the West Coast and was just beginning to feel better when a third attack of pneumonia killed him at 31.

He had frequented gay bars and bathhouses for anonymous sex. He told health investigators that his male sexual contacts numbered over 1,000 a year. Three months after his death his male roommate got AIDS.

A drug addict who was trying to kick the habit in a methadone treatment program became ill last April. He developed a rare cancer called Kaposi's sarcoma, and later died of the same pneumonia that killed the New York homosexual. He said he never had sexual relations with other men, but shared intravenous needles with hundreds of homosexuals.

A teacher in Port-au-Prince, Haiti, developed an infection characteristic of AIDs last winter, and is in a missionary hospital there. He told epidemiologists that he had never been a homosexual, drug addict or a recipient of blood transfusions.

A lifelong resident of Denver, 59, received frequent injections of a special blood product to treat hemophilia, a disease that increases susceptibility to prolonged bleeding. He gradually lost weight and was hospitalized last May with nausea, vomiting and recurrent fever. He died of AIDS in July.

A middle-aged southern woman moved north for a factory job and worked her way up to a white-collar post. Last spring she experienced shortness of breath, fever, cough and weight loss, later developing the form of pneumonia associated with AIDS. She said she had never used drugs, but her boyfriend did.

Two children of a San Francisco prostitute died from rare infections that weakened their immune systems. A third daughter also has a weakened immune system but remains alive. All three had different fathers. The mother, who also shows some symptoms of an immune system disorder, admits to drug abuse.

These stories, gleaned from painstaking interviews with AIDS victims' families, friends and doctors, suggest that the mysterious disease is spread by an infectious agent, much like hepatitis B, which affects the liver.

CDC officials say it appears to spread slowly by "intimate contact"--through bodily fluids such as semen or blood, or through close family contact--rather than rapidly, as a "subway-transmitted disease".

In 1981 the CDC began to get an extraordinary number of requests for a drug, available only through the government, to treat unexplained cases of rare Pneumocystis carinii pneumonia.

Five young homosexual men in Los Angeles, all previously healthy, had come down with Pneumocystis and been treated at three hospitals there between October, 1980, and May, 1981.

The cases were remarkable because this form of pneumonia usually strikes severely ill patients whose immune systems are damaged by cancer or by the drugs used in kidney transplants.

The cluster of cases was reported, with characteristic understatement, as "unusual" in the CDC bible of current diseases, the Morbidity and Mortality Weekly Report of June 5, 1981. On July 3, the cautious scientific description expanded to "highly unusual."

By then, 26 homosexual men--20 in New York, six in California--had developed cancer, Kaposi's sarcoma, in a unusually deadly form. Seven of them also developed serious infections and the number of rare pneumonia cases among them had grown to 15.

In the United States, Kaposi's sarcoma is an unusual but rarely fatal cancer, generally striking men over 60 of Mediterranean origin, and kidney transplant patients who receive drugs to suppress their immune systems and reduce the chance of organ rejection. It strikes more frequently in equatorial Africa, making up nearly 10 percent of all cancers there, where it commonly occurs in young children and adults.

Dr. James Curran, a young scientist in the venereal diseases control division of the CDC, began an emergency investigation.

"I felt from the beginning it was not going to be easy," he said. Within three months he was studying about 100 victims; now there are 10 times as many, with the end nowhere in sight.

Initially, the focus was on homosexuality. CDC researchers noted that "fast-lane" homosexuals--those who had numerous sexual partners, frequently used drugs and congregated in large cities--seemed particularly susceptible.

Nitrite inhalants--"poppers," dangerous drugs used in connection with sex--immediately were suspected as a possible chemical cause of the immune system breakdown. But the CDC soon found that homosexual men who did not have AIDS appeared to use drugs just as frequently as those who did.

A follow-up study showed that the AIDS victims averaged 61 sexual partners per year, compared with 25 for those in the control group, and the average lifetime number of sexual partners among AIDS patients was more than 1,000. The AIDS victims were also more likely to engage in sexual practices that drew blood, said Dr. Harold Jaffe, deputy director of the AIDS task force.

Tracking down this information was laborious. Jaffe said he checked into a hotel in San Francisco for $40 a night--all his government travel allowance would cover--and met through the day with a stream of young men who came to his room to be interviewed.

"The room clerk clearly thought something funny was going on," he said.

Eventually, doctors prepared an AIDS genealogy chart. There were sexual links between AIDS victims in nine of the 13 cases in southern California in which partners could be traced.

Intriguingly, said sociologist William Darrow, the "Los Angeles cluster" later was linked sexually with another group of 23 homosexuals in New York via a single, jet-setting man who had been a sexual partner with some men in both cities who later became AIDS patients.

By the fall of 1982 the sexual linkage list had grown to more than 40 men in 10 cities. There was a long latency period, from four to 30 months, between suspected sexual contact and symptoms of the illness.

One theory suggested that frequent sexual activity exposes homosexual men to a wide variety of germs, from hepatitis to herpes. Perhaps an accumulation of infections simply overwhelms the body, according to this theory, making homosexuals prey to other diseases.

Last year researchers also documented cases of what looked like mild AIDS, but they say they are unsure of their importance.

"Until you have an agent, if there is one, you don't know the extent of the disease. We may be looking at the tip of an iceberg of a severe epidemic, and we may also be looking at the tip of an iceberg of a disease that in most people is only causing mild symptoms and not this severe, irreversible disease," said Dr. Kenneth Sell of the National Institute of Allergy and Infectious Diseases.

In laboratories around the country, the search for the unknown AIDS bug continues, although no one is quite sure what they're looking for or whether it exists. Most of those involved in the AIDS search say they think it may be an entirely new organism or a virulent version of an old one.

"There are few new viruses under the sun," said Dr. Gary Noble, head of virology at the CDC. "But it may be a mutant of an existing virus with new properties."

Others suggest that perhaps a rare virus normally found in animals or in isolated areas outside the country simply found a new home here.

"The easiest explanation is an old bug that that has existed in some tropical country for a long time and has been brought into the country and amplified by life style," said the CDC's Dr. Donald Francis.

Speculation in the press focused on Haiti after a report last summer that 32 Haitians had come down with the disease in this country and that it appeared in New York and Haiti at roughly the same time.

Haiti is the poorest country in the Western Hemisphere, with high rates of disease and a shortage of doctors. Many Americans vacation there, and could have picked up a germ there, if AIDS is caused by one.

But poor countries are convenient scapegoats for the origins of diseases. Europeans long blamed Columbus for bringing back syphilis from the New World. New research suggests that the germ may have existed in Europe and coincidentally appeared at about the same time that Columbus returned from America.

Research targets include the herpes virus family as well as a newly discovered human T-cell leukemia virus, the first animal virus known to cause cancer in man.

National Cancer Institute researcher Robert Gallo recently reported that he had found signs of the T-cell leukemia virus in some blood samples from AIDS victims, but he was the first to admit that it was far too early to interpret the findings.

For researchers, AIDS is a disease that offers new prospects for understanding the way the immune system works and the links between infectious diseases and the development of cancer.

But for those who treat AIDS it is a frustrating disease that flies in the face of known medical therapy. There have been experimental attempts to stimulate the failing immune system with interferon, other drugs or bone marrow transplants. If an AIDS organism is found, doctors hope that a vaccine could be developed.

But, for now, "There is no reported case of anything being successful in reversing the immune defect," said Dr. Anthony Fauci of the National Institutes of Health allergy institute.

Others are seeking possible means of detecting the disease. Dr. Allan Goldstein, a George Washington University biochemist, reported last week on a possible hormone imbalance that might provide a simple test, but others say it is too early to tell whether it is specific enough for AIDS.

For the future, one government official says he worries that "the worst is ahead. There could be panic in the next six months or year." The CDC is on the lookout for possible new risk groups, particularly among families of AIDS victims and medical and laboratory staff working with AIDS. Employes at some institutions already are reluctant to work with the menacing disease.

"If it really is infectious and tranmissible through blood, ultimately everyone could be at risk," warned CDC's Jaffe. "It's certainly not going away. That's the one thing we're sure of."