Something worth keeping in mind: When living beings are possessed by Disease Spirits, other Disease Spirits are attracted and may congregate in the vicinity. In this manner, the appearance of a single Disease Spirit can result in an epidemic of Disease Spirits.
This is not an automechanical thing, but a “tends to”. So there are not game mechanics for this. Instead the GM is free to decide that possession results in no disease spirits showing up, or may decide that every day several more show up. Or even link the appearance of new spirits with the severity of the possession.
All diseases are caused by spirits, magic, curses, etc. I am not sure what a “naturalistic” disease would be.