In the developed world, combating HIV has been a triumph of modern medical science.

What was a universally fatal illness when it first appeared in the 1980s has become a serious but chronic condition, really not fundamentally different from diabetes. In fact, speaking to reporters on World AIDS day Dec. 1, President Bush felt it necessary to remind listeners of "those who have got HIV/AIDS here at home" - the more than 1 million people living with HIV in the U.S. - but the terror that an AIDS diagnosis represented 20 years ago has become a memory for those who can access and afford treatment.

In the developing world, however, HIV is part of a Gordian knot of poverty and disease (and often, political incompetence and/or unwillingness to face up to the disease) that makes it a much more formidable opponent. One of the many complicating factors is highlighted in the Dec. 8, 2006, issue of Science, in which researchers from Seattle's Fred Hutchison Cancer Research Center and the University of Washington show that the interaction between HIV and malaria fuels the spread of both diseases in parts of sub-Saharan Africa.

"We haven't looked at all of Africa," senior author Laith Abu-Raddad, told BioWorld Today. "But locally, the effects can be substantial."

Abu-Raddad said that his team set out to explain a paradox: HIV is spreading very rapidly in sub-Saharan Africa, but "measures of sexual behavior show that there is not so much high-risk sexual behavior over there, compared to other places. So one of the big puzzles is why?"

Circumcision "definitely" plays a role, and the prevalence of herpes simplex virus infections "likely" is a factor, Abu-Raddad said. But together they do not explain the rapid spread that is taking place. Abu-Raddad and his colleagues suspected that co-infection with malaria might play a role, and set out to test their idea using mathematical modeling.

The scientists used the town of Kisumu, Kenya, for their model. "We have malaria data, HIV data and sexual behavior data," Abu-Raddad explained. "So we used Kisumu like a laboratory."

The numbers are truly sobering. Abu-Raddad, an HIV/AIDS research scientist in the Hutchinson Center's Statistical Center for HIV/AIDS Research and Prevention and the Center for Studies in Demography and Ecology at the University of Washington, estimated that 5 percent of all HIV infections can be explained by the heightened HIV viral load induced by malaria. Conversely, his team estimated that 10 percent of adult malaria episodes in Kisumu are attributed to HIV .

In absolute numbers, that translates into 8,500 excess HIV infections and close to 1 million excess malaria episodes over the past 25 years in a single town with an adult population of about 200,000.

The reason malaria infections promote HIV transmission is that during malaria infections, the immune defense leads to a large number of circulating T cells, which are exactly the cells HIV targets. So during a malaria bout, "all of a sudden, [HIV] sees a huge amount of target cells," Abu-Raddad said. HIV viral load "goes up seven times. It's a huge increase, and it lasts for six to eight weeks." Transmission of HIV, in turn, is proportional to viral load, and so sexual activity in those six to eight weeks following a bout of malaria increases the risk that the virus will be transmitted.

Conversely, HIV-infected people are more susceptible to malaria infections (as well as any other infection) because of their already-compromised immune systems.

On Dec. 1, World AIDS Day, the European Union pledged $35 million for the development of an AIDS vaccine, while Canada announced it is committing $13.2 million to the International Partnership for Microbicides to support product development and clinical trials for women in developing countries.

But given the interaction of HIV and malaria that Abu-Raddad and his colleagues have demonstrated, it could also be counted as good news for those infected with HIV that in the first week of December, the World Health Organization unveiled the Malaria Vaccine Technology Roadmap - a strategic plan that outlines a pathway to the development of a malaria vaccine by 2025 that is 80 percent effective against clinical disease and would provide protection for longer than four years.