I suggest we all stop arguing and read the article referred to above. Here is another excerpt:
" ‘The immune system is there for a reason, said Dr. Kugathasan. "It’s there to recognize ‘the bad guys.’ The immune system allows your body to kill those bad guys and allows you to survive. In order to harden the immune system, the immune system requests some kind of stimuli all the time.’
" ‘The hygiene hypothesis suggests that the more hygienic one becomes, the more susceptible one is to various autoimmune diseases. The autoimmune diseases, the diseases that result from all the activation of your immune system, are increasing. The hygiene hypothesis - and we don’t yet have a proof of it - acknowledges that the maturation of the immune system needs some kind of hardening, some kind of resistance. Put another way, you cannot really build up good muscles without doing exercise.’
"From Pet Dander to Pig Worms
The common belief that has driven medicine, as well as public perception and hygiene practices, is that when we get sick it is because of something we ate, or inhaled, or were exposed to in other ways. The hygiene hypothesis points in a different direction, proposing that in many diseases it is a lack of exposure to the ‘bad guys’ that causes harm.
While the evidence was by no means clear-cut, one study indicated that in some cases contact with certain pet dander in the home actually decreases a child’s risk of wheezing from asthma later in life. Other studies show that children who lived on farms when they were very young have reduced incidence of asthma, which has led several researchers to conclude that organisms in cattle dust and manure may be the stimuli that their immune systems needed to fight off asthma."