I know when I'm cruising craigslist or back when I used to use alt.com and other dating sites I was a lot more worried about it. I'm a drug and disease free player so I know I'm not infecting anyone else but I was extra careful when meeting at public play spaces like the SF Citadel, etc.Now if I go to see someone they are usually someone with good references, good reviews on myredbook and such. These folks are as worried as I am about diseases and well educated to boot so I don't worry nearly so much. I did have a gal surprise me once by sitting right on my face mid session and forcing me to orally worship her or face suffocation. I didn't feel any plastic guards or dams but since I was well and truly strapped down to a table and being smothered I got to work and just had to deal with that niggling question in the back of my mind. But, again, I knew she was a pro so that helped calm me down.
As for fears of "crazy" people. Again, the professional nature of the folks I've played with mitigated it a bit. Oh the first gal I ever saw freaked me out a bit when she hoisted me up in a leather cage while I was in leather mittens and shackles. She said, "Well you're fucked now aren't you? Blindfolded, Shackled and restrained in leather suspended 3 feet off the ground in a cage in a strangers house where no one will hear you..." Boy that got to me mentally but that was a long time ago.
More recently I found myself hooded and bound then crammed into a cage. Feet chained outside and arm restraints clipped to the bars. Ms. then locked me completely in a leather hood with blinds and a gag, finishing it all off with some firm cock and ball bondage. She produced a note, saying it was from my wife, and claimed it was permission from her to keep me as long as Ms. wanted. The extra bit about it being from my wife (which was true btw) made it seem so much more likely that something had actually been worked out to that affect. Well this time it just made the whole thing hotter and not scary though, doh! So much for fear, LOL.
For a time when I actually did have fear ruin a scene... I was being tied to a St. Andrews Cross (was first time for me) and it seemed to just make me crazily sick to my stomach. Cramps, sweats, the whole nine yards. I had to safe word to get off since there wasn't anything outwardly or obviously wrong with me my top was just thinking I was role-playing. Luckily the space had a nice area for aftercare nearby and I laid down there for an hour or so till the feeling passed. The fear that my friend wasn't going to let me out while I had something honest to god medically wrong with me was really a turn off and I was pretty scared that something would really go wrong if I started contorting, throwing up and having cramps and such while tied up and unable to move to relieve the pain and tension!
I suppose we are lucky to live in a world where we can learn about SSC play, venereal and other diseases and have access to resources like myredbook on the internet at a moments notice.