Tuberculosis, a disease once gone from the popular consciousness and generally thought of as under control, is making a comeback -- primarily because of AIDS.

After more than three decades of steady decline, the number of new TB cases in the United States rose dramatically in 1986, especially in areas such as New York City where a high proportion of AIDS cases is found, according to the Centers for Disease Control in Atlanta. In 1984, there were 1,630 cases in the United States. In 1986, that jumped to 2,223, a 36 percent increase in just two years. Initial evidence suggests more of the same for this year.

The biggest increase in TB cases seems to be among patients with acquired immune deficiency syndrome, especially those who are minorities and contracted their disease through intravenous drug abuse.

While no one has declared a TB epidemic, infectious disease experts fear that the increase in active TB, especially among AIDS patients, will increase the risk of TB infection for the general public. Those most in danger include health care workers and family members -- including children -- and others living with and caring for people with AIDS.

"I think we have to assume the general population may be at risk," said Dr. Alan Bloch, chief of the surveillance and epidemiologic investigations branch in CDC's division of tuberculosis control.

The rise in TB appears to be yet another example of the complicated interaction between the human immunodeficiency virus that causes AIDS and other diseases. The virus attacks the immune system, shutting down the body's defenses against a host of other diseases, from rare opportunistic infections, such as Pneumocystis carinii pneumonia, to rare skin cancers and brain disorders.

An estimated 10 million Americans carry Mycobacterium tuberculosis, the cause of TB, in their bodies, but only 2,200 actually go on to develop active lung disease each year. If a person is already infected with TB and then becomes infected with the AIDS virus, the gradual destruction of the body's defenses will allow TB to grow, to change from merely being in the body to creating an active infection that can spread to others.

The increase in risk to the general population arises because AIDS increases the number of Americans who have active, infectious TB.

"The AIDS patient who gets TB will be spending some time with his family and friends, and unless we know that he has TB, it is transmissible to anyone who he shares the air with," said Dr. Lee B. Reichman, director of the pulmonary division at the University of Medicine and Dentistry of New Jersey in Newark and a TB expert. "It is not because he has AIDS that makes it more transmissible, but the AIDS makes it more likely that he will have the TB disease."

To become infected, someone merely has to breathe the same air as an individual with an active case of TB in the lungs and they too can get infected.

"This probably explains why the TB rates have risen," said Reichman.

The dangers may be greatest for health care workers who care for patients who have not been diagnosed as having AIDS or TB but who are nevertheless infectious. In such circumstances, the risk of contracting TB is much greater than the risk of being infected by AIDS. At the Carle Foundation Hospital in Urbana, Ill., for example, a dozen nurses contracted TB last year after treating an AIDS patient suffering from an undiagnosed lung infection. Unknown to the staff, the patient's constant coughing filled the air in his room with TB bacillus. As nurses walked in and out to care for him, they breathed air, pulling the TB bacillus into their lungs, and became infected.

Once the infections were discovered, the nurses were treated with an anti-TB drug; none of the nurses developed active TB disease.

The CDC has long recommended that health care workers treating TB patients protect themselves by wearing masks. The so-called BCG vaccine designed to protect against TB is not considered effective by American scientists, though it is widely used in other parts of the world.

The recent connection between TB and AIDS has emerged in a number of ways. "First, the areas of the country with the largest TB increases have also been areas that have the most AIDS," said CDC's Bloch. New York City, which has more AIDS than any other metropolitan area, has had a 35 percent increase in cases of TB from 1984 to 1986.

In certain population groups, moreover, the chances of getting TB with AIDS is higher. For example, foreign-born men -- from countries where TB is endemic -- who now live in Florida have a 20 percent to 30 percent probability of contracting TB if they become infected with the AIDS virus. Normally, a person infected with the TB bacillus faces a 10 percent chance of developing the disease over a lifetime.

A third link is that AIDS and TB tend to be discovered in a patient around the same time, Bloch said.

Tuberculosis itself may prove to be a harbinger of AIDS. It appears to show up earlier than many of the other odd diseases associated with this deadly epidemic and may be an initial indication that the person's immune system is failing.

The connection between AIDS and TB was first made when Reichman and his colleagues started seeing what he calls "weird TB." Most of the time, Reichman said, active TB disease is only found in the lungs. Sixteen percent of the time, it can be found in other parts of the body, including bone, brain and other organs, and even in the testicles.

In the AIDS patients, however, 75 percent of the TB was found outside of the lungs. "So we said, 'Weird TB suggests AIDS,' " Reichman said, especially if the patient has other risk factors for the disease, such as homosexuality or intravenous drug abuse. The prediction later proved correct.The Centers for Disease Control is currently considering modifying the definition of AIDS to include tuberculosis in a person infected with the AIDS virus.

TB usually can be successfully treated with a series of antibiotics, even in AIDS patients. Once treatment begins, the patient's ability to spread TB quickly ends, probably within a few days, Reichman said. It usually takes six months of treatment with three different drugs to cure active disease.

The risks of becoming infected with TB have been calculated, and health officials find that there is a 50 percent probability of becoming infected over six months of eight hours a day contact. This can vary depending on the intensity of the contact, the severity of the illness and how often fresh air clears the room of floating TB particles.

If an AIDS patient's TB is in his bones or some organ other than the lung, then he may be less infectious, Bloch said. Other studies suggest TB in AIDS patients is just as infectious. It is, he said, an issue that needs to be studied carefully.

Although the rise in TB and AIDS increases the threat of TB for everyone, the disease appears most among the poor and among minorities, especially those who use intravenous drugs.

"TB is more of a disease of poor people, and it is increasing with intravenous drug abuse," said John J. Seggerson Jr., chief of the program services branch of the CDC's Division of Tuberculosis Control. "Much of the homosexual population is white, male, middle-class and less likely to be infected with TB. Once AIDS infection reaches a group already infected with TB, the TB is allowed to arise."

The New Jersey research group speculated that if a given population already was widely infected with TB, then infections with the AIDS virus would bring that out. "Blacks or Latinos who do not have AIDS have a high rate of TB infection without disease," Reichman said. "When they get AIDS, or HIV infection, they get the infectious form of TB. And a community that has a lot of i.v. drug abuse in it also has a lot of TB."

In New York, there have been 261 cases of TB associated with AIDS. Nearly 60 percent were in intravenous drug abusers, Bloch said. The situation may turn out to be far worse than is already known. Of all known AIDS cases, only 4 percent have TB. But when Dade County, Fla., health officials consecutively tested 71 TB patients for infection by the AIDS virus, 31 percent were positive.

Said CDC's Bloch said. "I think the number of AIDS cases with TB that we are seeing is just the tip of the iceberg."