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The real world just refuses to cooperate with earmarks. That's the conclusion to be drawn from the article in the latest Economist highlighting the links between the natural histories of AIDS and malaria. The article drew attention to a study published in Science, by Laith Abu-Raddad and colleagues, estimating the impact on AIDS of malaria infection, and vice versa. With the TB-AIDS co-infection problem already very well established, this new work underlines a basic fact: The "cause of death" construct, translated into "burden of disease," and then translated into disease-specific earmarks and programs, doesn't conform very well to a world in which deaths tallied against one cause may actually be occurring because of another.

The Economist explains the AIDS-malaria link:

The increase in the number of virus particles [with malaria] is transient, and may do little harm to the individual's own long-term prospects, but it does make him (or her) more likely to pass the infection on during sex. Conversely, the damage HIV does to the immune system means that the malarial parasite can more easily breed unchecked. That means people are more susceptible to infection in the first place, and that more parasites are available to be transmitted from person to person by the mosquitoes that spread them.

For Kisumu, Kenya, for example, a model-based estimate of this interaction's impact "suggests the peak of the HIV epidemic is 8% higher than it would have been were there no interaction between the diseases, while the peak level of malaria is 13% higher."

No doubt we'll be hearing more about this in the coming months and years, and perhaps also about the many other co-factors that increase the toll of AIDS (and other diseases): malnutrition, other infectious diseases, and on and on. As evidence accrues about the complex interactions across causes of ill health, we're challenged to figure out how to break out of disease-by-disease thinking, and develop effective programs that simultaneously tackle multiple risks.