according to this: (source:
http://www.medicalnewstoday.com/articles/117536.php)
They are talking about couples where one already has HIV. So if you bang your girlfriend vaginally without protection 100 times, or for all intents and purposes one hundred women that HAVE HIV, you'd get HIV.
next, we have this: (source:
http://aids.about.com/od/womensresources/a/womenimpact.htm)
the population of the united states is just a hair over 300 million. If you assume half are chicks, then you got 150 million females, 300,000 of which have HIV. That is .2 percent.
Which means you'd have to bang, at random, 2000 girls to get one with HIV.
So if you take the first statistic, from that medical journal, where you'd have to bang your HIV girlfriend (vaginally only) a hundred times to catch HIV, and combine it with the second statistic, (assuming only vaginal sex, and all women are spread equally among risk factors) you'd have to bang a whopping
TWO MILLION CHICKS to have relative certainty of catching HIV.
Now, moving on to normal run of the mill STD's, you may go
here to calculate your odds of catching whatever it is you are worried about catching.