Plague Ship by Andre Norton — Read

Since Mura was in the isolation of ship sick bay the stripping of his cabin was a relatively simple job. But, though Rip and Dane went over it literally by inches, they found nothing unusual -- in fact nothing from Sargol except a small twig of the red wood which lay on the steward's worktable where he had been fashioning something to incorporate in one of his miniature fairy landscapes, to be imprisoned for all time in a plasta-bubble. Dane turned this around in his fingers. Because it was the only link with the perfumed planet he couldn't help but feel that it had some importance.

But Kosti had not shown any interest in the wood. And he, himself, and Weeks had handled it freely before they had tasted Groft's friendship cup and had no ill effects -- so it couldn't be the wood. Dane put the twig back on the work table and snapped the protecting cover over the delicate tools -- never realizing until days later how very close he had been in that moment to the solution of their problem.