Answered this morning.
I copied this answer from answerbag. I hope it helps:
There are so many ways to contract AIDS and HIV that its frightening when you think about it. Here are the most common:
1. Unprotected Sex - the #1 transmitter of AIDS and HIV, though that is being disputed currently. Unprotected Sex with anyone can lead to HIV, but also remember that it won't always lead to it. Generally, the rule with STD's is that one person is usually the infected carrier...regardless, always use protection when having sex. Most particularly, oral sex will DEFINITELY transmit the disease...the genitalia of both men and women are among the most germ-ridden areas of the body...oral sex will transmit the infection faster than most other methods, since there is no real contraceptive that can be used.
2. Drug Use - this is being disputed currently against Unprotected Sex as the #1 transmitter of AIDS. Sharing of drug needles is the most common transmission...needless to say, the needle goes into the blood stream of one person, and then if the needle is shared with other people, germs and infections from one person are spread to the other. While the direct cause of HIV is still not currently known(beleive it or not), it is a fact that sharing of drug needles seems to breed HIV among users.
3. Blood transfusion/mixing - It is never safe to touch someone's open wound...you don't know if the person has AIDS or HIV. A popular thing for kids to do way back when was to pick their scabs off and rub the sores together so that they would become 'blood brothers'. It is things like that which can cause AIDS...the direct contact of one person's open wound to an infected or potentially infected person' blood. In the 80's blood reserves were not checked for AIDS, since AIDS was still a young and relatively underestimated disease...many blood transfusions led to infection. Now, all hospitals and blood banks are required to screen donors for infections, but there is always the possibility that a blood transfusion will cause AIDS.
4. Kissing - disputed, but AIDS and HIV CAN be carried by saliva...kissing an infected person is dangerous. While a simple kiss on the cheek or non-mouth kiss is fine, open mouthed kissing can lead to the transfer of the disease. Also remember that when you brush your teeth, you create hundreds of tiny cuts on your gums...kissing an infected person who brushed their teeth recently is inviting a transmission.
5. Sharing hygiene tools - as stated above, sharing a toothbrush is a very bad idea...whether infected with HIV or not, the cuts created in the mouth of the person do bleed, and are open cuts...blood and infection will get caught in the bristles of the brush. Sharing shaving razors is also ill-advised...especially manual razors, as opposed to electric, since you are more prone to bloody cuts while shaving.
What you should remember is that you won't get AIDS or HIV by sitting next to someone with the disease, holding their hand, touching them or talking to them. Although it is easy to transmit, it is difficult to catch, but you must make sure that you are careful at all times...when you meet someone, namely a new friend or a date, it is important that you know their sexual history before you get romantically involved. If you trust the person, and they are open with their relationship history with you, then you can rely on that, but always remember that if you have doubts, ask your partner or friend (and you, for that matter) to get trested.