Women infected with malaria are more likely to pass HIV on to their unborn child, according to experts.
Mosquitoes are responsible for spreading malaria
The finding is based on a study of 746 HIV positive women in Uganda.
It found HIV was transmitted to babies in 40% of cases where the mother also had malaria. This compares to 15% for those without malaria.
Writing in the Journal Aids, the researchers said treating malaria could significantly reduce the number of babies being born with HIV.
Malaria kills over a million people a year. More than 90% of cases are reported in sub-Saharan Africa ,where it is the main cause of death.
Over 40m people around the world are infected with HIV. Rates of infection are highest in sub-Saharan Africa.
The transmission of HIV from mother to child is one of the main ways HIV spreads in Africa.
Dr Heena Brahmbhatt and colleagues at Johns Hopkins Bloomberg School of Public Health carried out a study to see if malaria infection increased the risk of transmission.
The 746 women involved in the study were tracked for five years between 1994 and 1999.
The researchers found that HIV positive women are infected more frequently and more severely with malaria compared to women who do not have the virus.
Women with both disease also had a higher risk of developing anaemia, miscarrying, having a low birth weight baby and delivering prematurely.
In addition, they had high levels of HIV in their bodies, which means they are more likely to pass on the virus to their child.
The researchers called for urgent trials to see if controlling malaria in HIV positive women reduced transmission rates to babies.
"Our findings show that co-infection with placental malaria and HIV during pregnancy significantly increases the risk of HIV transmission from mother to newborn," said Dr Brahmbhatt.
"Preventing and treating malaria during pregnancy could be a beneficial and cost-effective means of reducing the transmission of HIV from mother to newborn."
She added: "These findings could have potential public health relevance because interventions to prevent placental malaria during pregnancy might reduce the risk of HIV transmission from mother to child and this could augment current approaches using antiretroviral drugs.
"Randomized trials of intensive malaria control in HIV-positive women are urgently needed."