This means that while some people do show signs of STDs, the vast majority of people don't. Personally, I've never met a serious provider who didn't play safe and get tested on a regular basis. Keep in mind that not only are our bodies and our health quite literally our business, but in many first world countries, including several states within the US, knowingly transmitting HIV is a criminal (sometimes a murder) charge.
On the flip side, I have met MANY gentlemen who have not been tested in years. Most gentleman friends I have spoken to who have not been tested in a long while feel that since they are casual players, and their partners did not show any blatant signs of an STD, they were probably "fine." Let me repeat my original point here that most times, STDs are asymptomatic (at least until things get really bad, sometimes years later).
So, what's a horny boy to do?
1) Know which activities are considered high risk, and which are considered low risk. Obviously, avoid high risk activities (unless you don't give a flip about your heath or the health of your loved ones), and run like the dickens if a lady suggests engaging in a high risk activity. She's either poorly educated on the subject, or just doesn't care. Either way, if she's doing it with you, she's doing it with others. (Oh, and here's a hint-high risk activities include more than simply the oft mentioned "bareback."
2) Know your stats. Yes, herpes does hit at least 25% of the population. Some medical professionals estimate the infected population as much, much higher than that, but because they virtually never test anyone for this, unless an individual comes to the doctor with a current outbreak, the stats are unclear. However, most people also only have one or two outbreaks, and then the disease goes dormant (ie-no longer attacks the host or infects others, but stays in the system permanently.) However, also be realistic. The shit is a skin rash with a bad address.
Now, everyone's terrified of HIV, but if you wrap it up and play right, it's really hard to catch. Nevada has a whole host of legalized prostitutes that are required to use condoms and receive regular medical checkups. In the last article I read (admittedly written in 2005 or '06), in all of the years of this legalized prostitution with all of these ladies rotating through, not once did a lady end up as HIV positive.
So, what sort of diseases should you be worried about? Mostly the curable stuff that will completely and permanently destroy your bodily systems if left undetected and untreated. I've attached a photo of a mould of a patient's head in the tertiary stage of syphilis. Know what could have saved this sucker from looking like a poor imitation of the elephant man? Testing, and a shot of penicillin. That's it.
So, what's the moral of this story? While it's completely understandable that you want to play with clean partners (I mean, really, don't we all?), your best (only) bet understand the risks, play safe, and get tested regularly. As much as some guys seem to wish and bank on it, you don't get to abdicate responsibility for your health by putting the responsibility solely on providers to be clean. I can absolutely guarantee you that the ethical lot in here do our best (refer back to the part about our health being our business), but even in the civilian world, intimacy is never 100% guaranteed safe, and we can only do so much if the hobbyist side of the hobby isn't picking up their slack.
