The Sea Wasp (Ryk E. Spoor)

If you're a nomadic herder, you're leaving all that manure behind when you move on. No problem. If you've got a small farm (because your neighbors also have small farms with livestock), then you and your livestock and everybody's waste is right there, season after season.

As far as crowd diseases go, nomads might be in a pretty good position; they'd meet others often enough to get occasional exposure and thereby build up immunities, yet still have a good chance of being away when the big outbreaks hit. Someone must have studied this, I'd think.