Most workers in the field have observed a higher incidence in the young adult population afflicted in countries of higher socio-economic development whereas the incidence in older patients does not seem to follow this pattern and may obey the converse pattern (i.e. be more common in areas of lesser socio-economic development). Males are affected by the disease more commonly than females and in the ratio of 1.5:1.0. No one causative agent has been identified.
