Here’s an excerpt from Love and Math, a book by Edward Frenkel. The author writes about mathematics and his career. One of the stories is about how during his studies in the 80s he built a decision tree to help with kidney transplants. There was no machine to learn from data so humans had to do the work.
The third, and last, medical project I worked on was the most interesting one for me. A young doctor, Sergei Arutyunyan – who also needed help to analyze his data for a thesis – and I had a great rapport. He was working with patients whose immune systems were rejecting transplanted kidneys. In such situation the doctor has to make a quick decision whether to fight for the kidney or remove it, with far-reaching consequences: if they kept the kidney, the patient could die, but if they removed it, the patient would need another one, which would be very difficult to find.