There are many ways to die, but sometimes the cause of death is hard to pinpoint, especially before the modern science of forensics was established. Or further back when medical diagnoses were not always accurate, even the cause of natural deaths could be hard to determine. Then there are the cases in which the cause of death is pretty obvious, like a gunshot wound, but we never find out who did it or why.
The case of Edgar Allan Poe combines all those difficulties in the mystery of his death. Poe was only 40 years old when he was found incoherent and wearing someone else's clothing. He declined over several days before he died. The death certificate and his medical records were lost, although it was said he died of "brain congestion," whatever that meant. Some attribute his death to alcohol poisoning or alcoholism, or it could have been tuberculosis, or possibly murder by poisoning or something. In any case, Poe is only one entry in a mega-list of 25 mysterious deaths that still haven't been thoroughly solved at Mental Floss.