This World Is Taboo

Calhoun is an Interstellar Medical Serviceman, and he's needed on Dara. Trouble is: Dara is forbidden. Taboo. And breaking quarantine will make Calhoun a presumed plague-carrier and subject to being shot on sight by anyone from Weald. But hey! If he did the smart thing, we wouldn't have a story!
But why are men from Dara shooting at him?
(Summary by Mark F. Smith)