Upon casting, up to five initial parasites are created; one per creature in range. They will inflict 100 DPS up to a total of 300, and then move on to a random nearby creature. The parasite will die if it can't move to a new host. Each parasite can feed four times (five and six times for Plague I and II, respectively), for a total of up to 5*4*300=6000 damage. The extra feedings for Plague I and II bring the damage up to 7500 and 9000 damage respectively. Per second, this does only 100 damage per second per parasite. With all five parasites, it's 500 DPS which is decent, but a bit unexciting for a fourth-era Rarespell.
The central issue with this spell is that if all five initial parasites aren't created, the total damage is reduced by 1200 per lost parasite. Thus, it's important to target at least five initial creatures. Of course, it affects friendly creatures, so getting within range without infecting oneself is something to be careful of.
Infects up to 5 hostile or friendly units with a parasite that deals 100 damage every second. After 4 seconds the parasites pass over to the next available targets within a 25m range. Every parasite may affect a total of 4 hostile or friendly targets. Parasites will vanish if no more targets are nearby. Reusable every 30 seconds.