Germs influence societal structures

Societal traits may be influenced by how threatened we are by parasitic diseases:

Link to article

I remember reading once the theory that the caste system in India arose when the Indo-Europeans or some other group invaded the locals--the caste system was a way of keeping the germs from crossing caste borders.

I have also wondered over the years of parenting whether the availability of antibiotics influences how reckless we encourage our children to be. I started out really anxious (in parenting as well as in everything else) but then you see that children do seem resilient, and society in general encourages you not to baby them. But I've wondered if we would be less cavalier about allowing them to risk injury if each small injury had the capacity to turn into untreatable sepsis or something.

I don't understand why he talks about parasitic diseases only--would diseases caused by bacteria (which, it seems to me, are the ones that we worry about here in the U.S.) not fall into this model?