Guest: Paul Ewald.

Today on Focus, we'll talk to a challenger of the conventional knowledge that our genes and environments are the causes of most serious illnesses, including cancer and heart disease. Paul Ewald believes that these problems, among others—ulcers, miscarriages, schizophrenia, Alzheimer's—all come from infectious origins. He'll talk about what led him to this conclusion and his book Plague Time today on the show.