In studying the familial history of a family that was disposed to a certain rare genetic disease, we found that causes of death were all attributed to cardiac arrest, not the disease which caused it, until later years when the specifics were identified.
On a slightly related note, I've wondered that about Lyme's Disease. I was reading old death records (part of genealogy), and read about a young woman (in her 20's) who suddenly developed severe arthritis, and dies within months. Sounded like Lyme's Disease to me, and made me wonder how prevalent its been in the past.