In many public toilets there are disposable toilet seat covers that can be placed over the seat before sitting down. These covers certainly allay some of the "yuck factor" associated with placing one bare buttocks where so many have gone before, but is there any evidence that these covers actually prevent the spread of communicable diseases?
As pointed out by @KonradRudolph, this is only a partial answer. It addresses sexually-transmitted diseases, but not others. It needs further additions or a companion answer to properly address the question.
Before we can point to the evidence that covers help, we need some evidence that there is any problem to solve.
In a Salon article, Mary Roach investigated this claim. She spoke to experts from:
- reproductive endocrinology at Mount Sinai Medical Center
- American Social Health Association
- CDC’s STD hotline
- a microbiologist from University of Arizona
She also searched Medline.
She concluded there was no evidence of people catching any diseases from toilet seats.
Aurora Health Care also looked into this. They looked at a number of possible diseases, and quote a number of health organisations, such as the CDC, suggesting contracting a sexually-transmitted disease from a toilet seat is a "highly unlikely prospect".
They do concede at the end that it "can't hurt" to
take a few extra hygienic precautions when using public toilet seats. If you use a public restroom, it is reasonable to wipe off the toilet seat and cover it with toilet paper or a toilet seat cover before sitting down.
However, some measures may have negative effects. Women who avoid contact by crouching empty their bladders less.
The "highly unlikely prospect" however may have happened. A case-study suggests Neisseria gonorrhoeae may have been transferred to a small girl from a toilet seat, possibly in the act of wiping it down.
However it is also accepted that cases of non-sexual transmission of N gonorrhoeae in children do occur, but proof beyond all doubt can be very difficult to document scientifically
It references another article that explains in its title "Nonsexual Transmission of Sexually Transmitted Diseases: An Infrequent Occurrence".
A related issue is "toilet-seat contact dermatitis" which is an allergic reaction to the materials in the toilet seat, as opposed to an infectious disease.
In conclusion, the issue of whether toilet seat covers prevent disease is largely moot, because the risk of disease from toilet seats is so very low. (Oh, and crouching to avoid contact may not be a smart idea.)