Don't know where to start?
Knowledge is of two kinds: we know a subject ourselves, or we know where we can find information upon it. -- Samuel Johnson (1709 - 1784)
For an introduction to concepts and ideas for research topics, electronic reference sources or subject Encyclopedias can be useful.
Try the Subject Encyclopedias listed under Medicine and Health
For historical coverage of disease try: The Cambridge World History of Human Disease from Cambridge Histories Online Database