The Compound turns those who inhale it and the dead into zombies. There are several types of zombies, some more dangerous than others. Always keep your guard up regardless.
Revenant: Most common zombie. Body's not fully functional. Can turn others into zombies by biting or scratching.
Jackal: Undead dog. Generally not that tough but quite fast.
Convert: Someone who's breathed the Compound but hasn't died yet. Most dangerous at this stage as they're capable of using guns and other weapons to attack with. Also able to move at full speed since body hasn't decomposed. Should return to normal after a few hours of fresh air. If they've had contact with the undead though (e.g. bite or scratch) then their body will decompose meaning fresh air isn't going to help them (they need a cure).
Kamikaze: Undead birds. Named so due to their suicidal attacks. They usually attack in swarms.
Leaper: Large venomous spider that leaps on its prey. Can spin webs to capture prey as well.