July 4th 1826, Was the date that....?

The attached source from Monticello lists a diagnosis from a modern doctor, which is naturally based on historical accounts rather than actual diagnosis of the patient. In short it was "Exhaustion from intense diarreah, toxemia from a kidney infection, uremia from kidney damage, and orthostatic old-age pneumonia". The kidney problem seems to be related to a prostate problem. The doctor says that he might have lived longer, with modern care, but if this prostate problem was cancer, not much longer.

This next link diagnoses congestive heart failure for John Adams' cause of death. The information about his general health during his lifetime is very interesting, particularly his five day coma in 1781, while in Europe. He also may have had Graves' disease, which I also have, so that's of interest to me, particularly.

I read that they both died of Debility. Whether that is true or not, I do not know.

yes 50 yrs to the day both died

See below.