When eaten, it first takes hold within the stomach. The infested creature no longer has to eat, and indeed, cannot eat. Any food eaten after the first couple of days is quickly vomited out.
This is one of the few ways to detect an infested being in its early stages, short of cutting open their belly and searching for the mold within.
The infested sometimes know themselves as the Mold Vessels.
Within a 2d4 days, the mold has subtly lined every orifice and crept beneath the tongue. It stays hidden in this way for a further 4d6 days, and during this period the mold is transmissible via mouth-to-mouth contact, sexual congress, or by otherwise disgorging or digging out some mold and making another creature eat it.
After this, the mold can be contained no longer, and spreads out over the Vessel's skin. This is the penultimate stage, and such a creature is revered and protected by its peers.
Finally, it takes another 3d6 days for the whole body to be completely covered, eyes and all. A featureless soft furry blob in the shape of a man. Green-grey and lurid yellow. Black mold rimed with white. Silent on soft pads, quiet voice clogged with deep rich loam.
These are the Filled Men.
The Mold Vessels will obey any order and do any deed so long as it seems likely to end in the infestation of another being. They naturally congregate into cults beneath the most wilful of their number, for the infestation has no real effect on the mind of the infested creature save for a bone-deep, intense, whole-body rush of pleasure whenever they are involved in spreading the mold.
This feeling is shockingly addictive.