In the late 20th and early 21st centuries, developed countries became increasingly vulnerable to rampant immune system dysfunction, with ballooning rates of allergic and autoimmune disease. Why did this happen? A popular theory is that our hyper-clean environments, and the resulting lack of regular challenge to our immune systems – such as chronic parasitic infections – are causing our immune systems to misbehave. In this episode, producer Sam Ancona Esselmann sits down with Moises Velasquez-Manoff, author of An Epidemic of Absence, to explore this dramatic rise in autoimmune and allergic diseases and to discuss the caveats of controversial therapies.