I'm not sure if this theory has been discussed before, but I couldn't resist making a thread immediately after thinking about this. In fact I made an account just so I could share it.
The Dark King Silvers Raleigh tells us that Roger turned himself in once he learned that his life would soon come to an end. We are told that it is because of a untreatable disease.
Whitebeard was known as the only man who could rival Roger in power. During Whitebeard's meeting with Shanks, we see him hooked up to several IVs and what look like life support machines.
Would it be probable that Whitebeard and Roger contracted the same disease? The disease might have come from something near the end of the grand line where no other pirates had ever been before. Perhaps the disease comes from something that granted Whitebeard and Roger the power to surpass their rivals, but would bring them to their deaths?
Thoughts?