LOL, there are more MRI units in Boston than Canada proper. If you're happy with sociaised health care, what can I say?
Of the sexual transfer mechanisms, the highest probability of transfer is clearly that of unprotected anal sex. Anal sex between an HIV positive male and an uninfected partner, either female or male, can result in transmission of the virus once in every three to ten sexual acts (1/3 to 1/10). That rests in sharp contrast to the probability of transfer through unprotected vaginal sex from an HIV positive male to an uninfected female (1/75 to 1/300), or the opposite transmission from an HIV positive female to an uninfected male through unprotected vaginal sex (1/300 to (1/1000).
Oral sex is also a mechanism of transmission, but the small number of verified cases of such transmission, and the fact that oral sex is often accompanied by other forms of sexual activity, make it difficult to estimate a probability. There do exist verified cases of oral sex transmission, both heterosexual and homosexual; the probability is low, but quite real. A recent report indicates that persons with allergies, asthma, eczema, and allergic meningitis are more susceptible to orogenital transfer, presumably because of the excess of lymphocytes responding to allergy-induced inflammations in the mouth and throat.
Anal sex is a more likely mechanism of transmission than vaginal sex, primarily because that part of the body, unlike the vagina, is not evolutionary adapted for the physical activity of sexual intercourse. Simply put, the lower end of the large intestine (rectum) is a thin-walled tube that breaks easily, bleeds when it breaks, and has a major supply of lymphocytes and macrophages (including the CD4 group) waiting to be infected by HIV.
From the perspective of functional anatomy, unprotected anal intercourse is biologically stupid! It is an open invitation to serious damage of the rectum as well as to the easy transfer of any sexually transmitted disease such as gonorrhea, syphilis, herpes, papilloma and Hepatitis B. Hepatitis B (for which there is a vaccine) is more easily transmitted sexually than HIV, and results in the death of some 20%-25% of those who acquire an infection.
http://www.arcmesa.com/pdf/hivfun0100.pdf
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