The Papanicolaou test has been designed for early detection of a virus-caused cancer, but it also improves the protection against this cancer and against other diseases caused by sexually transmitted viruses.
This unexpected protective effect of the Pap smear has been found in South Africa where hormonal contraceptives and cervical cancers have been studied in more than fifteen hundred women. Besides pap smears for detection of the cancers and the human papilloma virus (HPV) that causes them, the blood of the women has also been studied in order to detect other sexually transmitted viruses. It came out that not only HPV but also Herpes and HI viruses have been reduced in women who have received the most Pap smears.
It seems to be a clear relation of cause and effect: The more Pap smears, the less viruses, and the more recent Pap smears, the better the protection. For instance, a Pap smear in the previous year reduces the odds ratio of being infected by Herpes simplex virus versus being not infected by 60 percent; this reduction is only 20 percent if the last Pap smear is ten or more years back.
Even for HIV, a protection has been observed but, of course, relying on Pap smears and not using condoms would be a very, very bad idea.
Being cautious also reduces the risk
A pap smear seems to protect against sexually transmitted viruses, but how? The study cannot answer this question. Indirect actions cannot be ruled out. For instance, a "cautious" or "careful" lifestyle may be linked to more Pap smears on the one hand and to less sexual partners on the other hand, and the number of sexual partners is one of the major risk factors for sexually transmitted diseases. Thus, more studies are required to find out if the protection is really due to the Pap smear.
Photo credit: flickr.com/photos/gregmce/164617348/