Experts say the research has limitations and is not proof, though.
Scientists at the University of California found evidence of a link between prostate cancer and the STDtrichomoniasis, in which a common parasite is passed on during unprotected sexual contact.
The parasite is believed to infect around 275 million people worldwide. Furthermore, over three-quarters of men harboring it have no symptoms and may not seek treatment, resulting in chronic inflammation of the prostate.
Working in the laboratory, the researchers found that the parasite makes a protein similar to a human protein needed by the immune system. The human protein has also been shown to be involved in the growth of cancers, as it causes inflammation.
The scientists conclude that infection with the parasite, along with other factors, might trigger inflammation that could lead to cancer growth.
More research is needed though, they say.
The study is published in the journal Proceedings of the National Academy of Sciences.
'No Clear Link'
Commenting in a statement, Nicola Smith, health information officer at Cancer Research UK, says: "This study suggests a possible way the parasite Trichomonas vaginalis could encourage prostate cancer cells to grow and develop more quickly. But the research was only done in the lab, and previous evidence in patients failed to show a clear link between prostate cancer and this common sexually transmitted infection.
"There’s been a lot of research into prostate cancer risk, and we’re working hard to piece together the puzzle. But there are still no lifestyle factors that seem to affect the risk of developing the disease – and no convincing evidence for a link with infection. The risk of prostate cancer is known to increase with age."
This latest research is not the first to link the infection with prostate cancer. A 2009 study in the Journal of the National Cancer Institute found that 23% of men with prostate cancer showed signs of trichomoniasis.