Centers for Disease Control & Prevention (CDC).
You can examine the Diseases & Conditions section for general information regarding a particular disease. For further information on various diseases, you can also refer to Medline Plus through the National Library of Medicine [http://medlineplus.gov/]. For statistics on diseases, see the section on Data & Statistics [http://www.cdc.gov/datastatistics/].
Contagion: Historical Views of Diseases and Epidemics (Harvard University)
This online collection offers important historical perspectives on the science and public policy of epidemiology today and contributes to the understanding of the global, social–history, and public–policy implications of diseases. These materials include digitized copies of books, serials, pamphlets, incunabula, and manuscripts—a total of more than 500,000 pages—many of which contain visual materials, such as plates, engravings, maps, charts, broadsides, and other illustrations.
MedlinePlus (National Library of Medicine)
Medline Plus is a great consumer health Web site. Look under the Health Topics tab to find articles about specific diseases, their causes, treatment, etc.
U.S. National Library of Medicine (NLM): History of Medicine
The History of Medicine Division of the National Library of Medicine collects, preserves, interprets, and presents materials documenting the history of medicine, biomedical science, health and disease in all time periods and cultures. It provides researchers and the public with access to manuscripts, books, photographs, films and other historical resources.
World Health Organization (WHO).
The WHO is a great resource for international health and disease information. You can search for information by specific Countries or Health Topics. Be sure to also examine publications and the Data & Statistics sections. The latter contains a resource called the Global Health Observatory, which is an interactive database bringing together core health statistics for the 193 WHO Member States.