[illumin8]

Frankly, I always considered it extremely rude and boorish to not remove shoes when entering someone's house (I'm american, if it matters). Think about it- would you drag an object through city streets, gutters, mud, sewage, and other bad areas, then drag it all over the floor of your house?

[gojomo]

Of course, if that's the host's custom, guests should follow it as well. But many hosts don't take their own shoes off.

While you never know what's on the streets, well-off areas of the US generally have clean, dry walkways -- it's rare for sewage to be tracked about. We have comparatively little dirt/mud/feces-transmitted disease -- such as typhoid, hepatitis, cryptosporidiosis, or various other waterborne parasites.

And, North American children face rising rates of allergies and autoimmune disorders, which, if the hygiene hypothesis is to be believed, are made worse by too little exposure to mild pathogens from unclean environments.

So the most health-promoting custom might vary a lot by region.