If you really want to know, you need to get educated. The Centers for Disease control has good information and some fairly reliable statistics.
http://www.cdc.gov/std/
Their HIV section disappoints me because unlike the other diseases, they obfuscate the stats so I don't know how many people in a hundred or a thousand might be infected.
However, other fact sheets are easy to read. Like the one on herpes:
http://www.cdc.gov/std/herpes/STDFact-Herpes.htm
Genital HSV-2 infection is more common in women (approximately one out of five women 14 to 49 years of age) than in men (about one out of nine men 14 to 49 years of age). Transmission from an infected male to his female partner is more likely than from an infected female to her male partner.
One out of five women in the US have herpes. That includes your mom, your wife, your daughter, your grandmother, etc... So the other thing to consider is "The Risk Group".
Teens are the number one risk group for most STD's last time I looked. Prostitutes and drug users, especially IV, were the next highest risk group.
Mikeishobby, you have to realize that the people playing here are a high risk group because of multiple sex partners. The group also is mixed with the other risk groups I mentioned. There are teen girls as well as many who are just out of their teens. Many of the people who play here are also drug users. Unfortunately, the guidelines prohibit talking at length about that in depth here.
The point is that the statistics in the hobby are most likely to be higher than the national average. If you are not assuming that everone has something, then you are probably being naive. That is just my opinion.
Stay safe.