Maybe you have heard about this before, but women, when selecting a mate, can subconciously smell the prospective mates immune system and it contributes towards or against the attraction, the womans senses are looking for an opposing immune system. The theory is that when it comes to caring for the kids only one parent will be sick at the same time.

In my experience it plays out that way... I can't remember the last time my wife or I both got the same bug, excluding the odd flu here and there.