In an age of “friends with benefits” and “quick hookup” apps like Tinder, the news that prostate cancer may sometimes be transmitted sexually should command some serious attention.

Evidence of this connection first arose in 2009 when a study showed that 23 per cent of men with prostate cancer were also infected with a parasite called Trichomonas vaginalis that is passed through sexual activity.

Is that a high number? It’s actually hard to tell because infections with this protozoan often produce no symptoms, so it is under-diagnosed. One U.S. study showed the prevalence of Trichomonas infections ranging from three per cent to 20 per cent, but that was in men attending clinics for sexually transmitted infections. They are probably not typical of the general population. We do know that, worldwide, this organism is the most prevalent cause of curable STDs.

Now, scientists at the University of California have figured out how Trichomonas vaginalis and prostate cancer might be connected.

Writing in the prestigious journal Proceedings of the National Academy of Sciences, Patricia Johnson and colleagues report that the protozoan secretes a protein that is similar to one found in the human body. That protein, called human macrophage migration inhibitory factor (HuMIF), is known to play an important role in the initiation and progression of cancers. The researchers report that the protein seen in Trichomonas infections, TvMIF, is 47 per cent similar to the human one. In biological terms, they found that TvMIF promotes inflammation and inhibits the movement of macrophages, an important component of the body’s immune system.

Their study involved taking blood from 111 people who had a Trichomonas infection, and 79 controls who reported that they did not have the infection. They first confirmed that the organism secretes the TvMIF protein in the body, since 57 per cent of the infected people produced antibodies to it, compared to 11 per cent of the uninfected ones.

The scientists then moved to the laboratory to study the effects of TvMIF on cell growth. They found that it promoted the growth and division of prostate tissue cells in a cell culture. Significantly, the increase was 20 per cent for normal cells and 40 per cent for cancerous cells. This is similar to the activity of the human analog, HuMIF. They also found that the presence of TvMIF increased the spread of both normal and cancerous cells by 30 per cent.

While it’s too early to suggest that prostate cancer is a sexually transmitted disease, this research fills an important gap in our understanding of some of the subtle links between human behaviour and cancer. We’re gaining an increasing appreciation of the role of protozoa, bacteria, and viruses in causing cancer.

A report in the journal Lancet Oncology notes that about one-sixth of all cancers worldwide are due to infections. A major one is cervical cancer, which has been linked to the human papilloma virus (HPV). Males can play a role in spreading HPV, which is why it is so important to vaccinate both boys and girls before they become sexually active.

Other infectious agents such as Helicobacter pylori and hepatitis B and C account for many cancers of the gut and liver. Again, vaccination is a powerful weapon, especially against liver cancers caused by hepatitis B.