Moises Velasquez-Manoff, author of An Epidemic of Absence, talks with EconTalk host Russ Roberts about his book--a discussion of why allergies and autoimmune diseases have been on the rise in the developed world for the last half-century. Velasquez-Manoff explores a recent hypothesis in the epidemiological literature theorizing the increase is a response to the overly hygienic environment in rich countries and the absence of various microbes and parasites. Velasquez-Manoff also considers whether reintroducing parasites into our bodies can have therapeutic effects, a possibility currently under examination through FDA trials. The conversation continues a theme of EconTalk--the challenge of understanding causation in a complex world.