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:
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)?