Gale Virtual Reference Library. Contains a wide variety of subject online encyclopedias all titles can be searched with one comprehensive search, or you may search within subject areas (example: Medicine).
Oxford Reference Online Premium. Provides a wide variety of subject online encyclopedias, dictionaries, handbooks, timelines, etc. Has a section for "Medicine."
Sage e-Reference Library. Covers a wide variety of topics/subjects and all titles can be searched with one comprehensive search, if you want. Also, includes a specific Health & Social Welfare section.