On World Tuberculosis Day 2007, the good news, according to the World Health Organization, is that the number of tuberculosis cases as a whole has leveled off.

But the number of cases of extensively or extremely drug-resistant tuberculosis, or XDR-TB, is increasing, prompting fears that tuberculosis not only will become a much bigger problem in the near future, but also that it will erase the gains made in fighting HIV in developing nations.

Paul Farmer, co-founder of Partners in Health, a medical charity whose goal it is to "bring the benefits of modern medical science to those most in need of them and to serve as an antidote to despair," told reporters at a press conference that "the rate of clinical response to a proper treatment regimen for AIDS is just excellent. And we know this from our work in Haiti and Rwanda that even in very, very resource-poor settings, people do extremely well - people with AIDS - when they're given proper therapy."

But, he said, those gains are in danger of disappearing in the nexus of HIV and TB infections. "It's extremely discouraging when you go from . . . [sound] claims of very successful programs . . . to a setting in which a substantial number of your patients are not doing well, not because they're not being treated properly for AIDS, but because they're not being diagnosed and treated properly for tuberculosis."

XDR-TB is what happens when multidrug-resistant tuberculosis, or MDR-TB, is not properly treated. And MDR-TB often is not properly treated.

The World Health Organization's description of how XDR-TB develops carries a faintly judgmental whiff: If first-line drugs to treat TB are "misused or mismanaged, multidrug-resistant TB can develop," they wrote in a press release on the occasion of world tuberculosis day Saturday. "XDR-TB can develop when these second-line drugs are also misused or mismanaged."

Jim Yong Kim, another co-founder of Partners in Health, had a more sympathetic description of how such mismanagement occurs in practice.

"People all over the world will do something for their loved ones to try to keep them from dying," he said at the press conference. "And what they do is some medicines here, some medicines there; they can afford three weeks of this medicine and four weeks of that medicine," which has led to XDR-TB from MDR.

At the press conference, Partners in Health was announcing a grant for $3 million from the Open Society Institute, founded by George Soros, to help health care workers in the southern African country of Lesotho more effectively treat people dually infected with drug-resistant TB and HIV/AIDS. Partners In Health and Boston-based Brigham and Women's Hospital plan to develop global treatment guidelines for the combination based on the results.

Lesotho was picked because its mix of high infection rates and limited resources are typical conditions of the front lines of TB control. As Farmer said, "Clearly the collision . . . between HIV and drug-resistant TB is happening in some of the places with the most scant resources to invest in public health."

Not to mention the rest of their infrastructure. Perhaps nothing was more telling about the challenges that developing nations face in delivering any sort of services to their people than the inability to get Lesotho's minister of health and social welfare, who was scheduled to participate, dialed into the conference: phone glitch.

Nevertheless, MDR and even XDR-TB are treatable - for now. The death rate is about 85 percent, but Farmer recounted a successful campaign to treat drug-resistant TB in the Russian penitentiary system: "In Tomsk Oblast in western Siberia, about a quarter of the people with tuberculosis in the prison systems were dying, and these were mostly young men. And once we introduced effective therapy for all TB, including drug-resistant TB, the death rates inside that prison went to zero."

Joanne Carter, associate executive director of the RESULTS educational fund, another medical charity, said that Partners in Health, together with doctors from Brigham and Women's, has treated "thousands" of patients with drug-resistant tuberculosis, some of whom had XDR-TB.

But, Kim pointed out, TB will not stay treatable if infection control remains inadequate.

"We worry that if we don't respond very aggressively now, we will soon end up with many patients who do have completely incurable tuberculosis," he said. "So, this response is very much focused on getting out ahead of this problem before we go back to the pre-antibiotic era in tuberculosis control. We don't want to get there, and the only way to not get there is to mount a very robust response immediately.

"I think mistakenly the world thinks that it has more time to deal with XDR-TB, but I think it's in error," Kim said.