Well like most of the posters have already mentioned, if you stick to the standard rub & tug service provided, I can't see you being at much risk so long as the girl has been properly sanitizing her hands between sessions, which is naturally in their best interest as well!
It's only those that seek out/receive extras you're not supposed to get at an MP that you may suddenly be putting yourself in a whole new risk category altogether, such as:
1) genital-to-genital rubbing (got that unexpectedly once, felt awesome but I smacked myself in the forehead afterwards for getting THAT carried away)
2) genital-to-anal rubbing (had one girl suggest I pump her between the cheeks, sort of like an "ass-cheek Russian" lol) Fun but still stupid given the proximity and all...
3) BJ / BLS - Not supposed to happen at an MP, never received a BJ there, but naturally a risk for oral to genital transmission (Herpes being the big worry for me, since that shit doesn't go away)
4) DFK/LFK - Some call it cold sores, I call it HERPES
5) Digits - If you're fortunate enough to engage in this taboo extra, remember where your fingers have been before you go dine on some finger-licking good KFC afterwards... lol
6) Prostate Massage - Some of us like a little back door play, and as such are inviting a whole potential heap of infections if that probing finger of hers ain't it's cleanest
That's about all the risks I've contemplated during my numerous MP visits... Still safer and more enjoyable for me than SPs though!
And one other key thing to note: we as hobbyists are always subject to some air of risk, be it (1) getting caught by our wives (not an issue in my case), (2) getting caught by LE with nothing better to do with their time, (3) getting scammed, or (4) catching something. Of the dozens of hobbyists I've chatted up over the years, most of us are more afraid of #'s 1 & 2 than #4.
All part of the game, I know I coin flip many a time, and if the girl's hot enough, well fuck it! lol
Safe and happy hobbying to all!
Raven out