Disciples are similar to both the Water Catalyst and Nova-6 Bomber zombies, buffing nearby zombies; unlike those enemies, however, they can heal themselves by draining the life of nearby players. Therefore, you must either take them out from a distance or go in with a strong enough weapon so that the amount healed is nullified. This is easier said than it seems with weapons like the Hauer and Gallo shotguns with the Dead Wire ammo mod equipped.