[quantum_mechani]

Sneaking units can never starve in enemy territory. The starvation they have encountered is a result of them attacking (the turn after they attack the starvation check occurs as they are in what is now their province). So, as long as they sneak into one of your provinces every other turn, they will never get diseased.

EDIT: It is difficult to tell for sure, but in general the starvation mechanics do often seem to operate as jimkehn describes.