The law, passed by the Japanese Diet in 1957, that Hersey discusses in the book was called the A-Bomb Victims Medical Care Law. The law was passed as a way of ensuring that victims of the bomb would have money to pay for their medical care. The law allocated money to be used to pay for this care.
The law designated four groups of people whose medical care would be paid for by the government. These included people who were in the city when the bomb fell, those who came into an area near the hypocenter in the next two weeks, those who had contact with bomb victims, and those who were fetuses in pregnant women who were in one of these categories.