Is it 100%? ie you have unprotected sex with someone with chlamydia, you get chlamydia too?
No, it's not 100%. But it's 100% your own damn fault if you knowingly have sex with someone with chlamydia (or any other STI) and end up with it yourself.
Because if the chances of getting AIDS through heterosexual sex are apparently millions to one,
Whoever or whatever told you that needs to admit they have no goddamn clue and stop spreading dangerous lies. Let's just get that out there.
The chances of contracting HIV through penetrative vaginal sex (which is what I'm assuming you mean by "heterosexual sex") are zero if your partner is HIV-, but much better than millions-to-one. Millions-to-one are the odds that jackpot lotteries bank on. There are, however, millions of heterosexual people dying of AIDS-related issues the world over as we speak. The chances of contracting HIV through penetrative anal sex are higher due to the nature of the tissues involved - but heterosexual people are no more or less likely than non-heterosexual people contract HIV through penetrative anal sex - or through penetrative vaginal sex, for that matter.
Anyway, it is difficult to create a meaningful per-act-risk number because of the many factors that can affect HIV transmission. Penetrative vaginal sex can quite easily be more risky than penetrative anal sex once you start factoring in the presence or absence of existing infections, abrasions, diseases, and so on. If you really want to know what the experts think the chances are, I suggest you do a search in a journal like the Lancet, or snoop through the CDC's information on it - but please remember that real-world conditions may vary, and if you don't wear a condom those odds you researched won't mean shit if you catch HIV.
what are the odds of getting chlamydia or other STI's?
Generally speaking, STIs other than HIV tend to be easier to catch. Most adults have one or multiple strains of HPV. I'm told that most adults have been exposed to and developed immunity to molluscum contagiosum. A large percentage (I don't have the numbers at hand) end up with herpes of one form or another by the time they pass away. To give you an example: My doctor informed me that in Canada anyone with an incurable infectious disease is supposed to be (in theory) reported to Health Canada by their doctor; however, HPV and herpes are so prevalent that they don't have to be reported.
Are they quite low or quite high? Maybe something to do with AIDS being a virus versus most STI's being bacterial?
Just for the infotainment of all who've bothered to read this far, HIV, HPV, herpes, the hepatitis family, and many other STIs are viral. Chlamydia, syphilis, LGV, and I'm sure many others are bacterial infections. Crabs are an infestation. AIDS is a syndrome, not a virus or a bacterium.
Basically I had sex with someone, the condom broke but I was only 'exposed' for a few seconds. She got tested and she was clean. So does this mean that I must also be clean or not?
If she tested clean you have a 0% chance of having contracted anything from her. It doesn't automatically follow that you are STI free, though. You may have caught something from another partner before her, but she just lucked out and didn't catch it from you. Nothing calms the nerves like getting yourself checked.
I hope I didn't come across as too much of a know-it-all, but I know there's a lot of false (and often dangerously misleading) information out there.
