A malaria drug rendered useless in most of the world by drug-resistant parasites is once again effective in the African country of Malawi. In a study in tomorrow’s issue of the New England Journal of Medicine, researchers report that chloroquine cured all but one of the 80 uncomplicated malaria cases it was used to treat in Blantyre, the country’s commercial capital.

Chloroquine was once one of the miracle drugs against malaria. Cheap, easy to administer, and with few side effects, it played an important role in eradicating the disease in southern Europe and the southern United States. But extensive use came with a high price. In the 1970s and 1980s, resistance that had arisen in South America and Asia spread rapidly across Africa, soon rendering the drug all but useless.

Malawi was one of the first countries to officially change its malaria policy, discouraging use of chloroquine by 1993. Few thought the drug would ever be useful again, but a few years ago, molecular analyses suggested that the mutation that confers resistance had nearly disappeared in the country, and preliminary studies of adults with asymptomatic malaria seemed to show that the drug could clear the parasite.

Is it any wonder, really? The mutations that render diseases immune to the drugs that are used against them generally make the disease less fit in an environment where the drug is not in use. If the drug is eliminated, and if a few of the parasites still exist without the mutation, these parasites will once again take over the general population. Just another example of how poor an example drug resistance is when used to imply progressive evolution.