How come the family members of two students that had H1N1 are not infected?

Brooke asked:


The two of them were from US to Malaysia. Back home to family and friends, few days later develop symptoms and confirm as H1N1 positive. Family and people in the same flight are quarantine.Been more than one week now but the family and other people in same flight are negative H1N1. H1N1 virus getting weaker?

Vaccine for H1N1