In addition to the fine books I mentioned yesterday, those interested in the history of disease might consider checking out Harvard's online collection, Contagion: Historical Views of Diseases and Epidemics.
The collection, which features over 500,000 pages of manuscripts, is freely accessible to anyone with an internet connection. The collection is broken up into categories, some of which focus on individual diseases (Syphilis, 1494-1923) while others gather sources on specific epidemics (The Boston Smallpox Epidemic of 1721) and still others are dedicated to broad themes (Concepts of Contagion). Each category contains links to scanned copies of manuscripts from Harvard's collections.
I particularly recommend the section called Pestilence and the Printed Books of the Late 15th Century. The Herbarius (1485) has such beautiful hand-colored pages. I wish I could read the marginalia.