Scientists have discovered that infections—produced by viruses, bacteria or parasites—can trigger some cancers. Insidious and symptomless, these infections fester quietly and produce cellular changes that can result in malignancies.

“Globally, 21 percent of cancers are linked to infection,” says the German Cancer Research Centre’s Dr. zur Hausen, who discovered the link between Human papillomavirus (HPV) and cervical cancers.

Cervical, stomach and liver cancers are the major ones triggered by infection.

Exposure to the common sexually transmitted infection trichomoniasis nearly triples a man’s risk of developing a particularly aggressive, lethal form of prostate cancer. Caused by a parasite—Trichomonas vaginalis—the infection is silent in women. It rarely causes symptoms in males, either.

A study in 1975 showed that liver cancer occurred almost exclusively in people who had been infected with hepatitis B —the majority of whom were carriers with no symptoms of liver disease.

Get tested (and cured) for any of these infections, or avoid them in the first place by using protection during sexual activity.

Find out more about infections that cause cancer.