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The spread of a particularly virulent form of tuberculosis in South Africa illustrates a breakdown in the global program that is supposed to keep the disease, one of the world’s deadliest, under control.

The program was intended to detect tuberculosis cases, make sure patients were taking their antibiotics, test patients for resistance to those drugs and monitor the spread of the disease.

But international tuberculosis experts say the system is in deep trouble for an array of reasons: misuse of antibiotics; other bad medical practices, like failing to segregate high-risk patients in hospitals and clinics; and cuts in government spending for such basics as adequate supplies of drugs and laboratories to do the testing.

Such factors have led to the rise of drug-resistant tuberculosis bacteria, a menace the world has only begun to appreciate.

Mycobacterium tuberculosis, the microbe that causes the disease, was discovered 125 years ago this month. Today, the bacteria infect 8.8 million people a year and cause 1.6 million deaths. They are spread in tiny droplets when patients cough.

Tuberculosis is curable, as long as the bacteria are susceptible to antibiotics. It becomes deadlier when it attacks people who are also infected with HIV, the AIDS virus. And when the tuberculosis bacteria become extremely drug-resistant, the death rate soars.

That was the case in Tugela Ferry, a rural town in KwaZulu-Natal province in South Africa, when an outbreak of extremely drug-resistant tuberculosis — XDR-TB for short — killed 52 of its 53 victims, all of whom were also infected with HIV. The outbreak was detected in 2005, but it did not receive international attention until it was reported at the international AIDS meeting in Toronto last August.

The World Health Organization calls the extremely drug-resistant form “a grave public health threat” because of its potential explosiveness among the millions of HIV-infected people in poor countries. It seems to be a lesser threat among people who do not have HIV, though it could be dangerous to the millions with weakened immune systems from treatment for cancer and other diseases.

XDR-TB is defined as tuberculosis that is resistant to the two most important anti-tuberculosis drugs (isoniazid and rifampin), along with two other drugs: a member of the fluoroquinolone class and at least one of three others (capreomycin, kanamycin and amikacin).

A step lower on the resistance scale is a form of the disease called MDR-TB, for multidrug-resistant tuberculosis. An outbreak of that form struck in New York City in the early 1990s, and cost at least $1 billion in emergency measures to control and manage tuberculosis patients.

Experts say the tuberculosis outbreak in South Africa is the deadliest one that they can recall.

Although South African officials, who have known of the outbreak for a year, promised a prompt and full investigation, even experts there acknowledge that efforts are lagging.

“Unfortunately, we do not know much more than a year ago” mainly because “a systematic survey in each of the provinces has not yet started,” Dr. Karin Weyer of the South African Medical Research Council told the Conference on Retroviruses and Opportunistic Infections here recently.