How did AIDS spread, exactly?

I don't understand how it could affect so many people if it was spread by er, merely fucking, given that it originated in Africa recently.

Also, if you run a business are you a "bigot" if you don't want to employ a guy who has AIDS? There is always a risk of it being passed on through saliva, urine, blood etc. Should people with AIDS be allowed to be free and spread their disease?