You may have used PubMed when looking for current articles about medicine, but it also includes articles dating back to 1946 (and the National Library of Medicine indexes articles going back to the 1870s in other databases--if you want these, contact me and I can help you find them). The older articles were considered secondary sources when they were published, in that people read them to gain scientific information. At this point, most of the science in these articles is outdated, but they can still be very useful for learning about the history of medicine. If you want to learn about what people in the 1950s believed about what cancer was, or how alcoholism could be cured, PubMed is a good place to look.
Note: Beginning June 1, 2021, users of NCBI services will need to login with their MSU Net ID instead of a NCBI-generated account. The NLM has more information about this change and how to link your existing NCBI account with your MSU Net ID. A Microsoft Word document containing step by step instructions is avaialble for download.