From summer 1942, the Auschwitz SS carried out regular selections of Jews on arrival. On average, around 80 per cent of new arrivals were selected for the gas chambers. Among them were the old, the young and the infirm, as well as mothers with younger children. The murderous decisions were made by SS officers like Dr Fritz Klein, a fanatical Nazi. Klein testified in Allied captivity in 1945 about his time as a Birkenau camp physician. He was sentenced to death by a British court and executed on 13 December 1945.
When transports arrived at Auschwitz it was the doctor’s job to pick out those who were unfit or unable to work. These included children, old people and the sick. I have seen the gas chambers and crematoria at Auschwitz, and I knew that those I selected were to go to the gas chamber. But I only acted on orders given by [chief SS physician] Dr. Wirths.[…]
to pick out those who were unfit
Dr. Wirths, when the first transport arrived, gave me orders to divide it into two parts, those who were fit to work and those who were not fit, that is those who, because of their age, could not work, who were too weak, whose health was not very good, and also children up to the age of 15. The selecting was done exclusively by doctors. One looked at the person and, if she looked ill, asked a few questions, but if the person was healthy then it was decided immediately.
Source: R. J. van Pelt, The Case for Auschwitz (Bloomington, 2002), p. 238