Histiocytosis is the name for a medical condition, when a human (or other animal) has too many hystiocytes. Hystiocites are specialized macrophages. Hystiocitosis is also used as a name for a number of diseases, which have this symptom. The diseases are relatively rare, they affect about one to two newborns per million. The WHO has classified the diseases in three groups. There are different forms of chemotherapy, to treat theose affected.