There's a story making the media rounds right now, based on an article in press at the Journal of Allergy and Clinical Immunology. In a nutshell: if women work on farms, or live with cats, while they are pregnant, the children they then give birth to are about 20% less likely to develop eczema. The results came out of a population study conducted by an international, mostly European group. Readable Reuters version of the story here.
One result is particularly interesting-- the finding that the children's risk of developing eczema dropped further with each additional species of animal to which the mothers were exposed. For three or more animals, the risk of eczema dropped to 50% compared to the average population. This "dose-response" is convincing evidence that the effect is real.
This supports the "hygiene hypothesis," that increasing numbers of people in the developed world are getting allergic disorders because they're not exposed enough to microbes when they're young. The hygiene hypothesis says that if you're exposed to bacteria etc. from pigs, cows, and sheep, the type 1 helper T cell arm of your immune system gets a good workout, and this suppresses the overactive type 2 helper T cell arm found in allergic disorders.
But: so what? This kind of research is neat, but you wonder what we are supposed to do with it. Not everyone has, or wants, the opportunity to muck out horse stalls so that their offspring have their chances of getting eczema reduced by 20%.
And cats--what's up with that? There's a lot of research out there showing that cat dander is a major allergen for eczema. Perhaps you should keep a cat until the moment of birth, and then give it away and vacuum the house thoroughly.
The study's authors (as one would expect of academics) make the caveat that "we did not study infectious diseases." So these kids who didn't develop eczema may have developed all kinds of other problems caused by microbes from animals.
This suggests that we need not abandon the hypothesis of good hygiene. I'm the dad of two kids under 4, and I know all too well where they put their hands. They need to be washed.