For the study, Evan Secor of the US Centers for Disease Control and Prevention and his colleagues infected macaques with schistosomes, parasitic worms that infect millions of people in Africa each year.
They injected infected and uninfected macaques with simian-human immunodeficiency virus (SHIV) via the rectum.
Only about 1/20th the amount of virus was needed to cause HIV infection in monkeys with worms compared with those without the parasite.
If infection with worms also increases a person's risk of acquiring HIV, and if this is true of infection via the vagina as well as the rectum, researchers said that it could explain why vaginal sex is riskier in Africa than elsewhere.
This could be because the urinary form of schistosomiasis, which affects up to 50 per cent of women in parts of Africa, damages the lining of the vagina, the first defence against HIV.
The study is published in PLoS Neglected Tropical Diseases.