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

Exposure to infectious disease in early life may have long-term ramifications for health and mortality. However, the ``hygiene hypothesis'' suggests reduced pathogen exposure may not be uniformly beneficial. 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 adult lifespan and morbidity. Pre-intervention surveys find widespread hookworm exposure among children in the American South, but minimal prevalence among adults, enabling separation of in-utero and childhood exposure. I show de-worming before age five leads to 2.5 additional months of life in a sample of adult mortality. Further, decreasing hookworm exposure is related to 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. Childhood de-worming leads to improvements in morbidity and mortality decades later.