When it's not as bad as a national catastrophe. Tyler Cowen links to a Washington Post story by Craig Timberg on how AIDS infection rates in most of Africa have been wildly overestimated:

Researchers said nearly two decades ago that this tiny country was part of an AIDS Belt stretching across the midsection of Africa, a place so infected with a new, incurable disease that, in the hardest-hit places, one in three working-age adults were already doomed to die of it.

But AIDS deaths on the predicted scale never arrived here, government health officials say. A new national study illustrates why: The rate of HIV infection among Rwandans ages 15 to 49 is 3 percent, according to the study, enough to qualify as a major health problem but not nearly the national catastrophe once predicted.

The new data suggest the rate never reached the 30 percent estimated by some early researchers, nor the nearly 13 percent given by the United Nations in 1998.

The study and similar ones in 15 other countries have shed new light on the disease across Africa. Relying on the latest measurement tools, they portray an epidemic that is more female and more urban than previously believed, one that has begun to ebb in much of East Africa and has failed to take off as predicted in most of West Africa....

Most of the studies were conducted by ORC Macro, a research corporation based in Calverton, Md., and were funded by the U.S. Agency for International Development, other international donors and various national governments in the countries where the studies took place.

Taken together, they raise questions about monitoring by the U.N. AIDS agency, which for years overestimated the extent of HIV/AIDS in East and West Africa and, by a smaller margin, in southern Africa, according to independent researchers and U.N. officials.

"What we had before, we cannot trust it," said Agnes Binagwaho, a senior Rwandan health official.

Years of HIV overestimates, researchers say, flowed from the long-held assumption that the extent of infection among pregnant women who attended prenatal clinics provided a rough proxy for the rate among all working-age adults in a country. Working age was usually defined as 15 to 49. These rates also were among the only nationwide data available for many years, especially in Africa, where health tracking was generally rudimentary.

The new studies show, however, that these earlier estimates were skewed in favor of young, sexually active women in the urban areas that had prenatal clinics. Researchers now know that the HIV rate among these women tends to be higher than among the general population.

The new studies rely on random testing conducted across entire countries, rather than just among pregnant women, and they generally require two forms of blood testing to guard against the numerous false positive results that inflated early estimates of the disease. These studies also are far more effective at measuring the often dramatic variations in infection rates between rural and urban people and between men and women.

That said, this is still unambiguously good news.