Knowledge (arcana), (nature), or (the planes) DC15:
Negative and positive energy can be found mingled throughout the Material plane in roughly equal amounts. Negative energy causes decay, wear, breakdown, and death; positive energy causes organization, healing, growth, and life. The lifeforce of a creature is a sort of unseen mechanism that harvests positive energy (or negative, if it's an undead creature) from the environment, though it doesn't generally work unless it's anchored to the body and controlled by a soul (or something similar but less potent, for nonsapient lifeforms). In order to stay alive, grow, and heal, living creatures generally maintain an imbalanced energy state with more positive than negative energy inside their bodies.
When a creature dies, its lifeforce breaks down and its soul departs (or is destroyed, drained away, etc, depending on its nature and cause of death). Either way, without lifeforce the body basically becomes an object. Unimpeded by the lifeforce, entropy gradually shifts the body into equilbrium with its environment by redistributing some positive energy out of it and more negative energy into it; this influx of negative energy is why dead bodies decay more rapidly. That same outflow of positive energy has to go somewhere, so most of it gets concentrated into its immediate environment. This is why dead bodies tend to fertilize trees and plants: they are basically drinking up the excess positive energy leaking out of the corpse as it strives to reach energy equilibrium.
The reason why corpses often spread disease is also related to this cycle: evil particles (which are the cause of diseases) tend to be attracted to concentrations of negative energy (because evil likes destruction). So when bodies decay and negative energy grows more concentrated in them, any evil particles in the area tend to be drawn to them, where diseases then manifest from the higher concentrations of evil.
