Andre Norton 
Plague Ship 

The tramp-freighter spaceship Solar Queen had exclusive trading rights to Sargol and its fabulous gems. But the crew’s bravery and resourcefulness strained to the breaking point as they met Sargol’s three challenges: the enigmatic obstinacy of the planet’s catlike natives, ruthless incursions of an illegal competitor, and worst of all — an invisible, undetectable stowaway whose presence branded the Solar Queen a plague ship . . . off limits to the rest of the galaxy!