If you have looked at older death records for ancestors, chances are you have come across a disease that you did not recognize. Some of the terms used were catch-all terms when the doctor wasn't sure exactly what the illness was. Others illnesses may be referred to as something different now.
Recently I found a nice free genealogy resource called Old Medical Terminology. The page is organized alphabetically starting with abscess and working its way down to yellow fever. In many cases, a brief description of the illness is given. Synonyms for the disease name are also given.