Google books has lots of things like the Lancet -- make sure you collect terms as you go, as the descriptions of what the diseases/conditions are changes over time.

(If you want an example of the kind of thing you can find, check out the link on my journal's sidebar to Squire's Companion to the British Pharmacopeia.)

And don't forget to hit your local library/medical library! Chances are the librarian knows sources you'd never even suspect exist.