There may be a link between malaria and the spread of the Aids virus across Africa, research by scientists working in Kenya suggests.
The study, published in the journal Science, says the way the two diseases interact can help them spread faster.
When people with Aids contract malaria, it causes a surge of HIV virus in their blood, making them more likely to infect a partner, the research says.
Meanwhile people weakened by HIV are more likely to catch malaria.
The diseases are two of the biggest killers in Africa.
Scientists studying the rapid spread of HIV-Aids in the city of Kisumu in Kenya found the spread of HIV was happening more quickly than they would expect just through risky sexual behaviour.
They investigated a link with malaria, which is prevalent in the area.
They believe that since malaria can multiply by 10 times the "viral-load" of HIV - the amount of HIV virus in an HIV-infected person's blood - the virus can be transmitted to a sexual partner more easily.
"This biological co-factor induced by malaria has contributed considerably to the spread of HIV by increasing HIV transmission probability per sexual act," said Laith Abu-Raddad, co-author of the study, carried out by the Fred Hutchinson Cancer Research Center and the University of Washington.
"In turn, the weakening of the immune system by HIV infection has fuelled a rise in adult malaria-infection rates and may have facilitated the expansion of malaria in Africa," said another co-author, James Kublin of the Hutchinson Center.
The scientists estimated that tens of thousands of HIV infections - perhaps 5% of the total - and millions of malaria cases - perhaps 10% of them - could be blamed on this co-infection.
The scientists said their findings had important implications for public health efforts - underlining the need for the authorities in Sub-Saharan Africa to tackle the two diseases together.
They also said it showed how other factors could influence the spread of HIV.
Dr Kublin said genital herpes and tuberculosis were also suspected of increasing the likelihood of infection.