The Fleabirds are a group of people indigenous to North America who have rejected the collectivism and sedentary lifestyle inherent in the Cosmic Era. Rather than living in the gleaming arcologies, steaming geofronts, or the great heaps of the urban rims and favelas. Rather, they live in caravans that travel the countryside of the continent.
A plague has hit the local area.
In humans it affects only the most vulnerable, the children and the very old, and even than it's little more than a summer cold. 24 hours of sniffles and then it's gone, barely noticeable really.
To sheep however it is fatal and the whole economy of the area is in serious trouble. If this keeps up the area could well be facing famine.
Somebody (enter the PCs) must find the rare herbs needed to make a cure.