gistfile1.java

@EventHandler(priority = EventPriority.HIGH)
public void zombieIsMasterRace(CreatureSpawnEvent e) {
    if (e.getEntityType() == EntityType.ZOMBIE) { // convert ALL zombies in ALL worlds
        if (ZombieSpawner.getSurvivorsZombie((Zombie) e.getEntity()) == null) {
            e.setCancelled(true);
            ZombieSpawner.spawnLeveledZombie(e.getLocation());
        }
        return;
    }
    final Entity entity = e.getEntity();
    if (!RUtils.isInInfectedWorld(entity)) return; // abnormal monster spawn only in infected worlds
    if (!(entity instanceof Monster) && !(entity instanceof Slime)) return;
    e.setCancelled(true);
    ZombieSpawner.spawnLeveledZombie(e.getLocation());
}

@EventHandler(priority = EventPriority.HIGHEST)
public void noVanillaZombiesWhatsoever(CreatureSpawnEvent e) {
    if (e.getEntityType() != EntityType.ZOMBIE) return;
    if (ZombieSpawner.getSurvivorsZombie((Zombie) e.getEntity()) != null) return;
    e.setCancelled(true);
    System.out.println("Blocked a vanilla zombie from spawning. This is a bug."); // fires whenever a zombie is spawned manually via command but not by egg
}