does ones' resistance increases with unhygenic environment?

Question

Ive seen people in the slum, drinking unclean water and eating unhealthy food. They do not become sick very often.

Answer 1

Its all about diet and exposure to bacteria. Regular exposure will "hone" your immune system into a lean mean fighting machine that is able to fight of infections. If however your immune system doesn't get the exposure especially when you are young, it won't be as good.

We are constantly under attack from microbes, and only a few actually get through, but early exposure will prepare the immune system.

Diet is important however, as the immune system uses lots of different vitamins such as A, D, E and C.