My back yard is forest. I'm no where near any busy streets and one of my cat's is 13. I never have trouble with them...they never get sick...just this one time with Murphy's absess. I don't know how I could change them from outdoor to indoor...almost seems cruel....But I understand the theory of keeping cats indoors. I suppose if I lived in a big city I would do it....but its all wilderness, even in Anchorage.