There are two primary reasons to collect family medical history. One is for your own and your family's health and well being. Having information of your family's medical history can be extremely useful to your doctor in his diagnosis and advice to you. You should list both of your biological parents and both sets of grandparents. Include on your list, birth and death locations, lifetime diseases and conditions, causes of death and ages at the time of death. Make a copy for your records and one for your doctor.
The second reason to collect family medical history is as part of your collective family history. Health matters could have had a great impact on your ancestors and for many following generations. Health issues were often behind important decisions in the family. For example, my grandparents decided to head west from Indiana due to my grandmother's asthmatic condition.
Collecting information about your family's medical history begins the same way as collecting historical data about your ancestors and family members. The first step is to ask questions of your immediate family members. Questions like, "What did Uncle Joe die of?" and "What kind of health problems did Aunt Jennie have?" A secondary source would be obituaries where the cause of death is often included.
If you look through enough obituaries of the 19th century you will eventually run across some peculiar medical terms. Terms like "apoplexy" which means a paralysis due to a stroke, and "consumption" which is Tuberculosis. There are many archaic medical terms which were used back in the 1800s that are not in use today. I discovered two web sites that have many of these terms defined: http://freepages.genealogy.rootsweb.com/~ilkane/OldMedTermsFrame1Source1.htm and [[http://loricase.com/old-medical-terms.html|http://loricase.com/old-medical-terms.html}}. They should help the researcher determine what happened to an ancestor.
Many statistics can be drawn up from a thorough medical history. One can compare the average body height, weight, and length of life for a particular century and compare it with national and regional averages. You can expand such a database to include eye and hair color, and study just how your own gene pool mixed. A medical history of your family can be quite dynamic and add an interesting facet to your family history.