The exchange of human organs for cash or any other "valuable consideration" (such as a car or a vacation) is illegal in every country except Iran. Nonetheless, international organ trafficking--mostly of kidneys, but also of half-livers, eyes, skin and blood--is flourishing; the World Health Organization estimates that one fifth of the 70,000 kidneys transplanted worldwide every year come from the black market. Most of that trade can be explained by the simple laws of supply and demand. Can you explain illegal trade in human organs using the simple laws of supply and demand?