Praxeon API

From their site: Praxeon Fingerprint.MD is a web service API for health and medicine. Fingerprint.MD can help your users to search using naturally phrased medical questions, stay current with news feeds, organize documents according to their subject matter, and recognize medical concepts within any document. For each concept, the concept Id, definition, and synonyms are available through simple API functions. Search supports the OpenSearch protocol, with search results in the Atom format.