It's not well understood how viruses can spread. For example, Magic Johnson's wife never caught HIV/AIDS even though she had unprotected sex with her husband for a long time.
Such example doesn't make sense because there is a perfect and scientifically proven explanation. About 1% of population is immune to HIV. They simply can't be infected by CCR5-utilizing strains due to mutation in the CCR5 gene.
http://en.wikipedia.org/wiki/CCR5#HIV