Hey Heather
I am going on what I have been told by The New Futures Project, but I had a quick look on google and this is what i found. Anal sex is classed as the riskiest activity as far as I am aware..
SAFE SEX - a knol by H. Hunter Handsfield, MD
Transmission of Primary and Secondary Syphilis by Oral Sex --- Chicago, Illinois, 1998--2002
Or-
During 2000--2002, of 962 persons with P&S syphilis, data were available for 627 (65.2%); 325 (51.8%) were MSM, 157 (25.0%) were heterosexual men, and 145 (23.1%) were heterosexual women. Overall, 86 (13.7%) persons indicated that oral sex was their only sexual exposure during the period they likely acquired syphilis: 66 (20.3%) of 325 MSM, 10 (6.4%) of 157 heterosexual men, and 10 (6.9%) of 145 heterosexual women (p<0.0001) (Table).
'Although it is possible to become infected with HIV through oral sex, the risk of becoming infected in this way is much lower than the risk of infection via unprotected sexual intercourse with a man or woman.
When giving oral sex to a man (sucking or licking a man's penis) a person could become infected with HIV if infected semen came into contact with damaged and receding gums, or any cuts or sores they might have in their mouth.
Giving oral sex to a woman (licking a woman's vulva or vagina) is also considered relatively low risk. Transmission could take place if infected sexual fluids from a woman got into the mouth of her partner. The likelihood of infection might be increased if there is menstrual blood involved or if the woman is infected with another sexually transmitted disease.
The likelihood of either a man or a woman becoming infected with HIV as a result of receiving oral sex is extremely low, as saliva does not contain infectious quantities of HIV.
More information can be found in our oral sex & HIV page.'
Of course, the only safe sex is with a condom.