This was a short story published in a collection of sci-fi short stories, at least as long ago as the eighties but probably a lot older, possibly even the fifties or sixties.
The gist of the story is that an astronaut is making first contact with aliens. They ask him to send a sample of flesh for research purposes. He sends it, then they return it to him (I can't remember why). The twist is, the astronaut knew all along the aliens were going to be hostile, and he sent them a sample of his cancerous tumor. The aliens infected it with a virus, thinking they were going to wipe out human life, but instead cured cancer. Hooray!
Anyway, if anyone knows what this story is, I'd love to read it again!