[Antonello]

Something I think is important to consider but didn't see mentioned in this article is the genetic lineage of the two groups of rats.

Obviously, most wild rats are descended from ancestry that have adapted to a dirty environment, but what of the lab rats? Were they from stock that has been bred in captivity for generations, or were they (or at least their parents) captured in the wild and transplanted into a 'clean' habitat?

I'd look into this possibility of genetic drift before I went off on a tangent tinged with Lamarck-like leanings.

[ahayes]

The next thing to do is capture wild rats and raise them in captivity for a few generations.

I tend to think that the hygiene hypothesis is true. Nothing Lamarckian about it, just environmental effects on the immune system.