AUTHOR=von Mutius Erika TITLE=The “Hygiene Hypothesis” and the Lessons Learnt From Farm Studies JOURNAL=Frontiers in Immunology VOLUME=12 YEAR=2021 URL=https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2021.635522 DOI=10.3389/fimmu.2021.635522 ISSN=1664-3224