In one of my games I found a province where I could build Kapa cheiftans (turtle guys with recuperation), when I placed a fever fetish on a one of them they got diseased but they never lost any hitpoints.
I wonder if the latent recuperation of the unit cured them of the disease and allowed them to go back to full health and they subsequently got diseased again on the beginning of a new turn.
The only bad thing is that I only got one firegem after 8 turns.