The US just approved a new treatment for a devastating disease that costs $2.1 million. It's the most expensive drug in the world.

The US Food and Drug Administration on Friday approved a cutting-edge new treatment for a devastating rare genetic disease called spinal muscular atrophy.  The treatment, Zolgensma, made by Novartis' AveXis unit, is a one-time therapy that works to treat the disease at the genetic level. That means the drug's effects should last a long time, though...