[Johann Mahne]

  The new rage is to let kids build up their natural immunity and let them play everywhere.
I once saw a dairy scientist drop a sweet onto the floor, she picked it up and popped it into her mouth.
She also smoked when pregnant.
  So i was wondering how effective her "resistance theory" would be if there was giardia,amoebic dyssentry,or cholera microbes on the floor.

[CliffordK]

I once saw a dairy scientist drop a sweet onto the floor, she picked it up and popped it into her mouth.

Oh, what Horrors!!!
And, she didn't even offer you a bite?

She also smoked when pregnant.

I can not advocate for ever smoking, except perhaps terminal cancer patients who believe some weed makes them feel better.

Smoking while pregnant is associated with increased probability of still birth, miscarriage and low birth weight.  Why take the risk, or risk the life of your future child?

Personally I think some people are too focused on hygiene, although I also realize it is not fun when one's children are sick all the time. 

Prior to the varicella vaccine, it used to be common to intentionally expose children to Chicken Pox because it was safer to acquire the disease as a child than as an adult. 

Many people believe that it is inappropriate to cleanse with antimicrobial soaps outside of the medical setting due to the risk of developing bacterial resistance to the antimicrobials.

Personally I've never had a flu shot, although perhaps the flu shot would be like "exercising one's immune system".  In a decade or two when I enter a high risk category, perhaps I'll re-think that.

Somewhere I think I read that farmers who are exposed to dirt and crap on the farm are at lower risk to developing disease than city folk.  Although, perhaps it is an unfair comparison as the diseases one would be exposed to would be different.

Ahh...  here it is.
Several studies indicating that farm kids have a lower prevalence of asthma and allergies (more from a web search of "Farm, City, Asthma, Allergies").
http://www.fyiliving.com/health/allergy-asthma/why-farm-kids-dont-have-allergies-but-city-kids-do/

Which you would think would be counter-intuitive due to the increased exposure to allergens.  In fact, perhaps it is the exposure to bacteria and allergens that provides some protection.

[Phil1907]

There is plenty of antigenic challenge in any human environment for development of healthy immune capacity.

[Geezer]

There is plenty of antigenic challenge in any human environment for development of healthy immune capacity.

What about poliomyelitis? I thought the outbreaks in the middle of the 20th century resulted from an improvement in the environment.