So begins An Epidemic of Absence, Velasquez-Manoff‘s new book about a tantalizing hypothesis for a modern medical mystery: Why autoimmune diseases, in which a person’s immune system attacks their own body, are becoming more common, even as infectious and parasitic diseases are beaten back. (Read an excerpt from the book)

According to Velasquez-Manoff and the scientists he writes about, it’s no coincidence. A fast-growing body of research suggests that immune systems, produced by millions of years of evolution in a microbe-rich world, rely on certain exposures to calibrate themselves. Disrupt those exposures, as we have through modern medicine, food and lifestyle, and things go haywire.

Velasquez-Manoff, who has several immune-related disorders, including food allergies and alopecia, had heard about the “hookworm underground” — people who infect themselves with parasites in the hopes of restoring immune balance. Though it’s something he now recommends against doing, it marked the beginning of a reportorial journey into a frontier of science and health.