In regards to statistics, some studies have shown that as many as 1 in 12
gay men in London are HIV+ compared to 1 in 20 gay men in the rest of the UK, or 1 in 330 people in general in the UK. [more statistics
Particularly in the early stages the tests can be a false negative, which is why HIV tests are repeated at intervals after known exposure. That said, the risks of getting HIV from donated blood are essentially zero (I don't think there's been a known case of it happening).
In regards to blanket-banning all unprotected sex, I believe the rates are much lower for ladies (including
homosexual ladies) and hetrosexual men. Quoting that; "To date, there are no confirmed cases of female-to-female sexual transmission of HIV in the United States".