Doctors in India say they have found cases of a rare strain of tuberculosis that appears to be totally resistant to antibiotics.
They say that for two years they tried to treat a dozen patients from Mumbai's slums, but the infection proved drug-resistant.
Experts say similar cases of virtually incurable tuberculosis have emerged in Italy and Iran, and the new cases pose a serious threat to global efforts to control TB.
TB is one of the world's biggest killers, second only to HIV among infectious diseases, the BBC reports.
Normally a patient is given a six to nine month course of antibiotics to eradicate it, but strains of the bacterium have developed which are increasingly resistant to the antibiotics most commonly used to treat it.
Partially drug-resistant TB is found in countries across the world and "multi-drug resistant" strains affect countries such as Russia and China.