Hunter's take sacred vows drinking the blood of an undead. They must never fall in love, never have children, never own land and never abandon the quest of killing all undead. If you break the sacred vows, you will be hunted by another Hunter and killed without mercy. The drinking of blood stops all feeling throughout their bodies taste included. Hunters are taken either from slavers, adopted from orphanages or even stolen from non-willing families. They are trained from birth to do one thing: hunt beings that are an upfront to the gods, those include undead, liches, demons, devils and most evil creatures.
Following the code and taking the vows gives you an immunity to pain and fright. Hunters have great knowledge of evil creatures which can be incredibly inciteful.