Dirtier Lives May Be Just the Medicine We Need by Matt Ridley on WSJ.

Your great-grandparents faced infectious diseases that hardly threaten you today: tuberculosis, polio, cholera, malaria, yellow fever, measles, mumps, rubella, smallpox, typhoid, typhus, tapeworm, hookworm…. But there's also a long list of modern illnesses that your great-grandparents barely knew: asthma, eczema, hay fever, food allergies, Crohn's disease, diabetes, multiple sclerosis, rheumatoid arthritis. The coincidence of the rise in these "inflammation" diseases, characterized by an overactive immune system, with the decline of infection is almost certainly not a coincidence.
Natural experiments in recent decades support the idea that while modern hygiene defeats infection, it also promotes allergy and autoimmunity. Finns isolated in an impoverished Soviet province had more parasites and fewer allergies than Finns in Finland. Swedes in clean Stockholm had three times as much asthma as Estonians in smoky Estonia. Ethiopians and Gambians got allergies when they lost their intestinal worms. Growing up on a farm greatly cuts allergy risk.

In a remarkable new book, "An Epidemic of Absence," Moises Velasquez-Manoff draws together hundreds of such studies to craft a powerful narrative carrying a fascinating argument. Infection with parasites prevents or ameliorates many diseases of inflammation. The author briefly cured his own hay fever and eczema by infecting himself with hookworms—before concluding that the price in terms of diarrhea and headaches was too high.