you have had sex with all the people your partner has had sex with (idea)

For HIV, this slogan is rather misleading - it is usually more relevant how many times you've had intercourse than how many partners you've had. The transfer rates for HIV are low - 1/600 for a girl having intercourse with an infected boy, 1/800 for a man having intercourse with an infected woman.

Of course, this is no reason to be careless - though the odds are low, the cost ending up on the bad side of them is horrible, not just for you, but for your friends and family, not to speak of any partner(s).