Plague Ship by Andre Norton

Lured by exotic gems and valuable oils, the crew of the space trader Solar Queen landed on the newly discovered planet Sargol only to find their most ruthless competitor there ahead of them. Still, they tried for fair trade, even according to teh sly rules of the strangely feline, native Salariki. But after takeoff they found they had a plague on board, the treachery of someone who'd been on Sargol. Although they'd begun to run out of supplies, they couldn't land or they might start an interplanetary epidemic . . . and the galaxy had ordered that the Solar Queen be destroyed on sight!