The hygiene hypothesis postulates that the absence or delay of certain infections, primarily in childhood, results in an increased risk of allergic and autoimmune conditions.1 Exposure to intestinal colonizers, from microbiota to parasitic organisms, plays a role in the maturation of the immune system and these organisms may have coevolved with us in a way that makes classification as routine parasites untenable.2 Urbanization and decreased exposure to particular potentially infectious organisms have been implicated in the increasing prevalence of allergic and autoimmune disease.3 As a consequence, some of these symbiotic relationships may be more accurately considered as forms of mutualism because of the benefit that accrues to the host. The presence of these organisms is not associated with significant morbidity and may cause negligible mortality. Therefore, their decreased frequency under heightened hygienic conditions has been implicated in causing a dysregulated immune response.4
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Ontaneda D, Cohen JA. Keep the Worms in the Mud. JAMA Neurol. Published online June 15, 2020. doi:10.1001/jamaneurol.2020.0519
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