Early-life infectious disease exposure, the 'hygiene hypothesis', and lifespan: evidence from hookworm.

Abstract

Exposure to infectious disease in early life may have long-term ramifications for health and mortality. This study leverages quasi-experimental variation from the Rockefeller Sanitary Commission's de-worming campaign in the early 20th century, combined with pre-campaign hookworm prevalence, to rigorously examine the impacts of childhood hookworm exposure on later-life morbidity and lifespan. Pre-intervention surveys find widespread hookworm exposure among children in the American South, but minimal prevalence among adults. I show exposure to de-worming before age five leads to 2.5 additional months of life in a sample of older-age mortality. Further, decreasing hookworm exposure is related to later-life declines in biomarkers for inflammation and skin-tested allergies, in contrast with the predictions of the ``hygiene hypothesis''. Placebo tests using health outcomes that should not be affected by de-worming do not show similar patterns.