According to the hygiene hypothesis children today have more allergies due to an overly clean environment that results in the immune system overreacting to nonpathogenic substances. This can be prevented in part by living on a farm or at least having a dog in the house. The is also credible evidence that probiotics (supplements or fermented foods with beneficial bacteria) play a role in reducing allergies.

Both my wife and myself have had worms when we were children and neither of us have any serious allergies. Even though I have had asthma since I can remember (other kids complained about my "noisy" breathing) it was never diagnosed until I was in my mid 30's (when a record amount of pollen and fungal spores made it severe enough for me to see a doctor). In other words, my condition is annoying but nowhere near life-threatening. 

My son's asthma is much worse than mine and a blood test revealed that he was allergic to almost everything. In addition to normal asthma therapy, using a HEPA vacuum cleaner, and avoiding certain foods I give him sublingual immunotherapy (a cheaper and milder alternative to allergy shots) and probiotic supplements. Two years ago we achieved our goal of keeping him out of the emergency room and this spring was even less scary, so we are hopeful it's working. We can't keep a dog, but I never discourage (almost encourage) my children to play in the dirt. I know this sounds crazy, but when we returned from a trip to Mexico and my son's butt was itching I was actually hoping he had contracted pinworms (to whip his immune system into shape) but it turned out to be just some minor irritation. 

It seems that this hypothesis also applies to autoimmune disorders. This makes sense because allergies and autoimmune disorders both involve the immune system attacking non-pathogenic substances. The hypothesis is being tested in a clinical study using whipworm parasites to treat people with Crohn's disease! Will this eventually result in "parasite pills" or "petting zoo regimens" for urban and suburban children? If these things are no longer part of your normal life (as they are for children who grow up on farms or in less developed countries), being exposed artificially is preferable to living with life-threatening allergies or autoimmune disorders. Sign me up!