Scientists have known for some time that the immune system suppression caused by HIV can increase both the risk and severity of malaria infection.
But the idea that malaria might fuel the transmission of HIV is more recent.
Malaria sufferers often experience repeated, nonlethal outbreaks of the disease's flulike symptoms, with episodes varying in severity.
Studies have shown that in people infected with both diseases, the amount of HIV virus in their bodies goes up significantly during these malaria episodes.
Other research has demonstrated that as the amount of HIV virus goes up, so does the likelihood of HIV transmission through sexual intercourse.
These two factors together can cause HIV to spread more rapidly in populations where malaria is present.
"Malaria cannot be the only reason why HIV has so predominantly affected sub-Saharan Africa," said James Whitworth, a scientist not involved in the study, of the medical research nonprofit Wellcome Trust in London.
"But it is certainly plausible that is has been an important cofactor in driving transmission."
The discovery of a significant HIV-malaria interaction suggests the need for a coordinated approach in fighting both diseases, scientists say.
"It highlights the need to integrate health programs," said Jonathan Mermin, a physician with the U.S. Centers for Disease Control and Prevention working in Kenya.
"People with HIV should be provided with insecticide-treated bed nets and [anti-malaria medications]," Mermin said.
"Efforts to prevent and eliminate malaria should be increased, alongside efforts to prevent and treat HIV infection."
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