I like this view, thanks for the link. Hep A was listed as a proxy for hygiene in that study, so the inverse correlation with acute lymphoblastic leukemia was not directly causal. Hep A itself likely doesn't prevent or treat leukemia, rather something about low hygiene conditions does. Maybe some common bacteria or other virus. The core point stands, we live in and depend on a messy biome, in particular ways we don't yet understand precisely.