Edward was likely tubercular, but it was inactive until he caught the measles.
Or, if you subscribe to the theory, Northumberland poisoned him with arsenic, which in is deadly, but when ingested over a long period of time, actually
prolongs life rather than shortens it.
Now, my apologies if I'm derailing the thread but I was thinking today at work about this:
Edward marries Jane, but dies before they can have children (because let's face it, they may be only fifteen but if he's already sick it mightn't happen as quickly as it should - and perhaps it doesn't, or they do, but she doesn't get pregnant before he dies).
From my post in a different thread:
Northumberland marries the dying Edward VI to Jane. Thus, after Ned kicks, unless it can be proved that the marriage wasn't consummated (which many will see as Catholic propaganda anyhow), Jane gets some breathing room until she has her courses. Of course, this keeps Jane on the throne - only as holder of the Crown Matrimonial - (unless Edward's published his Device for the Succession) for a bit longer than nine days.
How does this affect things? Can she be considered a well-known enough figure to be proclaimed queen in her own right. And if Parliament's ratified Ned's device, who will support Mary against Jane (besides maybe the Habsburgs)?