A hundred and twenty-five years ago, a virtually unknown German country doctor called Robert Koch stood before the Physiological Society of Berlin and announced that he had discovered the cause of tuberculosis. This was probably the most astonishing and significant statement in the history of medicine. Yet the disease he discovered still kills 1-2 million people a year and new strains of the TB bacillus threaten to undo the progress of 125 years.

TB was rife in the 19th century and responsible for about one in seven deaths. The list of the famous who fell victim to the disease, including Keats, Chopin, and the Bronte family, gives a measure of its impact. Trained as a physician, John Keats knew the significance of the drop of blood coughed on to a bed sheet: "That drop of blood is my death warrant. I must die." And his prognosis was accurate - he died within a year.

So Koch's claim made headline news around the world and offered hope of a cure. Early in the 20th century, Paul Ehrlich (who had TB himself), led the search for "magic bullets". However, it wasn't until the 50s that the antibiotic streptomycin was shown to be capable of killing the TB bacillus. But problems emerged in the shape of resistant strains. Trials sponsored by the medical research council showed effective treatment required a combination of drugs over a six-month period.

Six months is a long time, particularly in the developing world, so it is no surprise most deaths from TB today are in Africa and Asia. The HIV/Aids pandemic has increased the level of infection as the virus makes victims more susceptible to TB. In 1993, the situation deteriorated to the point where the World Health Organisation (WHO) declared TB a global emergency; thanks to its efforts, the incidence of disease has since levelled off, and in some places has fallen.

But extensively drug-resistant TB (XDR-TB) is threatening to undermine these gains. The first sign of TB fighting back came in the 90s, when there was an outbreak in New York of TB that was resistant to normal frontline drugs. There were scores of deaths and more than $1bn of spending was needed to bring it under control. But spending at that level is not an option for developing countries. The town of Tugela Ferry in KwaZulu-Natal, South Africa, recently experienced an outbreak of XDR-TB among HIV-infected people. Of the 53 victims, 52 died of the disease, and their average life duration was 16 days.

XDR-TB is a product of inadequate treatment, and the key to managing it is improved infection control and new drugs. But lab resources remain basic in poor countries, and although research funding for TB has increased, it is still dwarfed by spending on other, less immediately real threats. Smallpox hasn't killed anyone for decades but, because of its association with bioterrorism, it receives as much research funding as TB. Most westerners see global warming as a much bigger threat. The Global Plan to Stop TB, an international partnership backed by the WHO, would cost an extra $1.1bn in 2007, a fraction of the cost of implementing the Kyoto agreement on carbon emissions or the £26bn to replace Trident. The cost of providing antiretroviral drugs for the world's estimated 6 million Aids victims in the world would be about $1.5bn.

Drug-resistant TB is already common in Asia, and some eastern European countries have the highest rates of XDR-TB. Cheap travel and increased migration ensure that it will spread. If we fail to act now, says Paul Nunn, coordinator of Stop TB, we will be faced with the "need to solve a human catastrophe, at vastly greater expense".

· Johnjoe McFadden is professor of molecular genetics at the University of Surrey and an editor of Human Nature: Fact and Fiction

j.mcfadden@surrey.ac.uk