I'm not an expert on this subject, and I won't quote you alot of useless statistics, but if each of your 20 partners have had 20 partners of their own, that equates to you having had sex with over 400 people. Unprotected sex in this day and age is very risky business. The best advice I would offer (although unsolicited) is to be more careful in the future and always carry condoms. Getting tested now and staying disease-free will be worth the effort.

Slightly higher with each subsequent partner, and by the way, it's the 21st Century.

The odds are meaningless. If you have unprotected sex with anyone you are playing Russian roulette with your life. It does not matter how clean the person is or if they say they do not have a disease. Cleanliness and current lifestyle does not mean they do not have HIV. People with HIV can be perfectly healthy to the eye. They only become visibly unhealthy as they go into AIDS. HIV is the virus, AIDS is the actual illness. Since you are a straight male, then I assume your partners are women. I have bad news for you: women, straight, lesbian, or bisexual, have just as much of a chance of contracting or carrying HIV as any other person. Gay sex is NOT the only way to get it. Also, with an incubation period of years there is no way for her to know if she has it right NOW. All she, or you, or I need to have done is had sex with someone who is HIV+ ONCE. You and your partner (partners?) need to go get tested. I am a bisexual male who has, because of my extreme precautions, remained HIV and STD free my entire life. I am one of those people who, when entering into a sexual relationship, insist that my parter and myself are tested for HIV together BEFORE entering into a sexual relationship. I am also tested for HIV every 6 months when working in direct patient care and every 1 year when not. -------------edited to add: I wish to add that, after you have passed the HIV test with your parter, that does not mean that you are free and clear. You could seroconvert the day after you are tested. Seroconversion is when you go from HIV- to HIV+ status. That is why I have the strict regimen of HIV tests for myself. I think it is the only "right" thing to do. It is how I protect others from anything I do not know that I have. You also need to be checked for HEPc and HEPd. I think there are more HEPs out there now but I am not up on it.

The odds are pretty high. Straight or not always use a condom. Because in the end you can only blame your self if you catch something, if you don't wear one. She may not know she has something when she has sex with you. And If she's having unprotected sex with you, she may have with others as well.

fairly high - I would estimate > 30%. Statistically the probability does not increase with each partner as a previous responder suggests - but it just takes one. Equally, you are not playing Russian roulette as another responder suggests - HIV is really a chronic disease now that is well managed by drugs, certainly not a death sentence despite the rhetoric to the contrary. Having said that - stay negative if you can.