Witches and witchers are people with special powers and abilities from birth. The church traditionally considers them to be heretics who communicate (in different ways) with monsters.
The most powerful witches and witchers demonstrate their forces literally from birth, which is why they die - fearing bad consequences for the whole family, parents often kill such children themselves.
Others can live in ignorance to old age, until a strong emotional shock provokes a spontaneous release of magic.
Those who succeed in escaping often join one of the clans - mountain, sea or forest. Each newcomer, depending on his abilities and inclinations, becomes under the care of an older witch/witcher, who take care of and teaches to cope with their own strengths and weaknesses.