The AIDS epidemic is fueling a sudden and alarming worldwide increase in tuberculosis, the ancient scourge that has been one of history's deadliest diseases.

The problem is worst in parts of Africa, where the spread of AIDS has left hundreds of thousands of people newly vulnerable to TB. But health officials also report a sharp increase in TB in the United States, reversing a decline that had continued for decades. Tuberculosis, which was the nation's leading killer at the turn of the century, had been all but forgotten since the 1930s.

The return of TB -- a bacterial infection that can be fatal if left untreated but which is curable with antibiotics -- complicates efforts of health care workers around the world to cope with the spread of AIDS, stretching already strained resources. It also demonstrates how insidious the AIDS virus, HIV, has become, fomenting consequences that could not have been imagined when the epidemic began.

"The irony of this to me is that the disease most feared at the end of this century is causing a recrudescence of the disease most feared at the beginning of the century," said Harvey Feinberg, dean of the Harvard University School of Public Health.

After declining in the United States from 80,000 cases a year in the early 1950s to just over 22,000 cases in 1984, TB began to rise again in the late 1980s. Last year it jumped 5 percent nationally, with urban areas with high numbers of HIV-infected people showing dramatic increases. In Newark, for example, TB cases were up 35 percent; in Los Angeles, 32 percent. Among young blacks in New York City, a group with a very high rate of HIV infection, TB cases are up 150 percent.

The resurgence of TB stems from the fact that the disease, even though it had been successfully controlled in developed countries in recent years, never really went away. Almost 10 million Americans are infected, and a large number have carried the disease for years.

In a healthy person, however, the chances of TB infection developing into full-blown or active TB -- the only type that is either infectious or dangerous -- is slender. What AIDS has done is to so weaken the immune system that HIV-infected people are more likely to pick up TB if exposed to someone with the disease or more likely to develop active TB if they themselves are carrying the TB bacterium.

In this sense, TB is like any of the other opportunistic infections that prey on HIV-infected people when their immune systems become damaged. However, TB strikes unusually quickly. While most opportunistic infections cannot successfully attack an AIDS patient until his T-cells -- the central player in the body's immune system -- drop from a normal level of 1,000 per cubic milliliter of blood to 200, the TB bacterium attacks sooner, when the level has dropped to about 350.

Unlike pneumonia or cancer, the most common causes of AIDS death, and unlike HIV itself, TB can be transmitted by casual contact, a fact that poses an additional headache for public health officials.

TB experts stress that this risk of transmission is not overwhelming. When the TB infection settles in the lungs, as it does typically, it is in a place from which it can easily come into contact with others when the infected person coughs. But to have a 50 percent chance of catching TB, public health experts estimate, a healthy person would have to be exposed to someone with active TB eight hours a day for six weeks. For reasons not exactly clear, more than half of all AIDS-related TB cases are not infectious, because the TB bacteria settle in another organ and not in the lungs.

Health officials also stress that even when TB is transmitted and develops into full-blown disease, it can be treated effectively through a simple course of drug therapy.

"TB is 100 percent preventable and 100 percent curable," said Lee Reichman, a TB expert at the University of Medicine and Dentistry of New Jersey.

Although the cost of drugs for TB is relatively low -- about $70 for a full treatment -- officials say the epidemic could not have come at a worse time and that the human resources needed to treat and screen for the disease are in many cases simply not there.

"This is not a problem in Iowa or Nebraska," said Philip Hopewell, a TB expert and professor of medicine at the University of California at San Francisco. "This is important in New York and Miami and San Francisco. If you look at areas of the country where there is a heavy concentration of HIV-infected people, there has been a striking impact on the health care system."