Why the hell is there fresh food in ruins that have been sealed for thousands of years? This has been seriously bugging me. Then I thought, you pass a tree stump. It has mushrooms growing on it. You play for a couple of in-game years, and the same stump has the same mushrooms. this got me thinking.
One of the Et'Ada died, and became one of the Bones of the Earth. It's influence : things fall into Nirn, hence we get gravity.
Another Et'Ada died, it's influence became Nirn turns, facing the sun, the hole Magnus tore in Oblivion, so you see the magic of Aetherius, the day, then facing the dark night that is Oblivion in turn.
Another died, and it's influence was the body of anything descended from the Ehlnofey decays.
There was not, however, any Et'Ada that became one of the Bones whose influence was that vegetable matter, or flesh removed from a now soulless body, decays. A dead mammoth rots to it's bones, an apple or a chunk of meat cut from the rest of the mammoth does not, as there is nothing in the laws of Mundus telling it to. Disease and decay are not spread by similar agents, as in our universe.

Nonsense of the highest order, but who has a better idea?