As with many health questions, articles that discuss the history of a disease or treatment might be an important source of data. PubMed (see Module 3) is an excellent starting place. You can use the "Limits" section to select the History of Medicine "Subset", or you can use the MeSH Database to do a subject search by a particular historical period (as in History of Medicine, Medieval). Of course, bear in mind you are unlikely to find the full-text of the article here (unless you are willing to pay for it).
Researchers would also want to investigate books on the topic. The National Library of Medicine is the largest medical library in the world and has the largest history of medicine collection. Their online catalog, LOCATORplus, is a good online starting place. After that, using your own library's online catalog will help to determine if they own the book you need. Again, the full-text of books is unlikely to be available from library catalogs. You will need to visit the collection in order to read the actual book.
Another useful resource is the Index Catalog of the Surgeon General's Library, which allows the researcher to find citations for articles, books, and dissertations from the 1800s (and before!).
Often, libraries or museums will mount online exhibits about the history of a disease. These can be invaluable resources to begin your investigation. One example that provides a wealth of information about yellow fever is the Philip S. Hench Walter Reed Yellow Fever Collection at the University of Virginia, which offers access to both primary and secondary information (via the Web).
The media (television, radio, film) can also provide useful information. PBS, which frequently does documentaries on health topics, often mounts a Website that complements the content of their aired programs. The Website for the PBS documentary on the 1918 Influenza Pandemic is an excellent example, using illustrations, maps, and quotations to give an overview of the effects of this devastating event.