In the story, explorers sent to a certain planet were susceptible to a native disease, to which Earthlings have no cure. There is a legend that an early explorer who caught the disease got stranded away from base for a while and had to subsist on a native rat-like animal; when he finally got back to base, he no longer had the disease. (The veracity of this legend is controversial.) Earth scientists work feverishly to replicate this cure. The story describes the various experiments that are performed and all the different approaches they try, none of which are successful. Finally, a non-scientist explorer thinks to try replicating the original legend directly. He eats a rat raw, as the original explorer would have had to, and the cure works. Only then do scientists discover in hindsight that the cure was in some compound or micro-organism that gets killed or denatured by the preparation processes the scientists put their samples through before experimenting.
I read this 40-50 years ago, so my accuracy is somewhat approximate.