In medicine, histiocytosis is an excessive number of histiocytes (tissue macrophages), and the term is also often used to refer to a group of rare diseases which share this sign as a characteristic. Occasionally and confusingly, the term histiocytosis is sometimes used to refer to individual diseases. According to the Histiocytosis Association, 1 in 200,000 children in the United States are born with histiocytosis each year...