2) Likewise, you say that for a man to catch HIV with unprotected vaginal intercourse with a woman one time, even if she were HIV positive
, is like 1 in 1,000 (or 1 in 2,000?). Again, medically, what is it about the disease that makes this so rare? Also, just curious, where do these numbers come from? Again, from the medical/biological perspective, what is it about the HIV virus that makes this so?