My stab at it.
The magnetic field from the body has 3 components: 1) an intrinsic contribution from the magnetic moments of the magnetic atoms (like iron, chromium, etc.) and 2) the magnetic field that results from changing intrinsic electrical activity (firing of neurons, etc.); 3) magnetic fields generated by induced electrical activity due to exposure to external electric or magnetic fields (e.g. MRI scans, lightning, etc.).
Maxwell's Equations tell us that electrical currents give rise to magnetic fields.
Quantum Mechanics tell us how to calculate the magnetic fields from atoms.
There's nothing mysterious about the magnetic field from a living body - AFAIK. When it dies, the contribution from the magnetic atoms remains. The contribution from the intrinsic electrical activity ceases when the intrinsic electrical activity ceases.
In each case, the fields are very, very small, compared to the magnetic field of the Earth.
Cheers,
Scott.
