South African President Thabo Mbeki famously asserted that poverty -- not HIV -- causes AIDS. Mbeki's claim may have been dangerous pseudoscience (one Harvard University study blamed his government's AIDS policies for at least 330,000 deaths), but he was right that socioeconomic factors play a major role in the transmission of the virus. Poverty does affect the spread of HIV, but in a more complicated way than was previously thought.
A recent study by Ashley M. Fox of the Mount Sinai School of Medicine compares rates of HIV infection across 170 regions in 16 sub-Saharan African countries. Surprisingly, she found that in the poorest regions, it was richer people who were more likely to be infected with HIV, while in wealthier regions, the poor were more at risk.
The reason, she argues, is that AIDS acts more like a chronic condition, such as obesity, than the infectious disease it is. In Ethiopia, for example, the wealthy are more likely to be overweight, a result of more affluent lifestyles and a greater array of food choices. In the United States, however, the poor are less educated about nutrition and eat lower-quality food, increasing their obesity risk. AIDS, says Fox, works in a similar way. Wealthy people in developing countries tend to have more sexual partners and are also more likely to be mobile, traveling to cities for work and coming into contact with the virus. The effect, however, tends to even out once the disease becomes more common, as the wealthy adopt safer habits -- while poorer people, with less education and access to condoms, are more vulnerable.
Fox's research focused only on developing countries, but it might hold true in richer countries as well. In the 1980s, AIDS in the United States was often described as a uniquely "democratic" disease, affecting all social classes, but today it's considered a disease of the poor. In low-income urban communities, Americans living below the poverty line are twice as likely to be infected, according to a recent Centers for Disease Control and Prevention report.
In the three decades since it was identified, AIDS has gone through a remarkable socioeconomic mutation, from a condition closely identified with gay men in urban areas of the United States to one synonymous with poverty in the developing world. Fox's data suggest that despite more than 30 million deaths over the past 30-odd years, it's still a disease we don't understand very well.