Death certificates are public records in most areas. In some areas they won't release death certificates for 10 or 20 years or more. So they're available for older, deceased relatives. The death certificate will list the cause of death, as well as complications to the cause of death. Older death certificates do not always use current medical terms.
Researchers can find medical dictionaries in the library as well as some on the Internet.
And researchers should not only look for death certificates for parents and grandparents, but also aunts and uncles and cousins and grandparents' brothers and sisters. For situations due to unnatural causes of death, there are coroner's records, sometimes called medical examiner records. There are also hospital records which are sometimes a little more difficult to obtain, but they should be sought out. There's several organizations that researchers can look to for guidance in their genealogical research. One is the National Genealogical Society.