Depends on what you mean by passive. Like an aura that affects all nearby enemies or a random enemy every N seconds? The unit with the ability acts as if parasite was cast on it all the time?
The aura wouldn't be too hard. Just make a dummy aura that affects enemies, and when a unti dies, check if it has the buff the aura gives, and if so, make a watery minion thing. If you want it to act just like a summoned unit, you could make a dummy caster with a regular summon ability (like summon bear). For random enemies nearby, every N seconds pick a random enemy nearby that doesn't have the Parasite buff, make a dummy caster, and have it cast parasite on that unit.
If you want the unit to just spawn a minion on death, go ahead and do that with a trigger.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.