From Technic Wiki
The Zombie Syringe can be used to instantly grow baby animals or Villagers into adults, in the same way Growth Syringes do.
It can be used either by hand, or in a Veterinary Station which will automatically use a Syringe on any baby animals with a 5x5 area in front of it. Empty Syringes are returned after use and can be refilled to use again. Unlike it's counterpart (the Growth Syringe), the Zombie Syringe has a downside, occasionally turning the baby animal into a grown Zombie.
The Zombie Syringe, when used on a villager, will turn them into a Zombie Villager.