Let's see if I can help puzzle this out...
Hmm... Okay, to understand this we have to reference the Steal Shadow ability of the shadow undead monster. This may clarify:
Steal Shadow <> (divine) Requirements The shadow hit a living creature with a shadow hand Strike on its previous action; Effect The shadow pulls at the target's shadow, making the creature enfeebled 1. This is cumulative with other enfeebled conditions from shadows, to a maximum of enfeebled 4. If this increases a creature's enfeebled value to 3 or more, the target's shadow is separated from its body (see shadow spawn). The enfeebled value from Steal Shadow decreases by 1 every hour.
The basic pattern seems to be the shadow spends 1 action on a strike, and then if that strike is successful, they can spend 1 action to Steal Shadow causing them to become Enfeebled. If they gain Enfeebled 3, the monster takes their shadow as per Shadow Spawn and creates a minion from it.
The modification in Familiar Shadows is that instead of pulling the target's shadow free and creating a shadow spawn, it applies a debuff 'infesting' the target until either the target is Dying and recovers, or until the shadow infestation curse is removed in the same way as normal curse removal.
The text doesn't explicitly say that the infestation only starts if the shadow can drag you down to Enfeebled 3, but I think that's reasonably implied. However, given the line about the curse, it seems like recovering from Enfeebled does not end the infection the same way it would for shadow spawn.