There's a very nice story in Discover Magazine, about Dr. James Allison. He's not an M.D., but rather a PhD biochemist and immunologist who discovered the principle of the "checkpoint inhibitor" - drugs that can activate the immune system to attack cancer, even though the cancer cells are not foreign invaders. The story is available at:
The people who work at this kind of research can spend decades trying to isolate and possibly modify a single molecule, figure out its structure and function even though it's too small to see even in a microscope, and test its activity, first in a test tube, then in mice, and finally in humans with cancer. Along the way, they learn things about the nature of cancer and of human biology that nobody knew before and that become the starting point for further and deeper research.
We rarely even learn the names of people like Dr. Allison but I think people like him are heroes who make a real difference in our lives.