> exercise our immune systems This is a widely held but incorrect belief. The immune system does not work like a muscle - it doesn't atrophy when unused and it

> exercise our immune systems This is a widely held but incorrect belief. The immune system does not work like a muscle - it doesn't atrophy when unused and it doesn't become stronger from repeated challenge. The immune hypothesis is about specific kinds of pathogens - parasites and bacterial microbiota[1] that humans co-evolved with and have specific defenses against (such as IgE antibodies). The exact mechanism(s) have yet to be determined but it's clear that lack of exposure during early childhood increases risk of autoimmunity. Other than that, getting exposed to a larger variety of pathogens - especially viruses - just means you get sick more frequently[2]. The frequency of exposure to modern human viruses are a relatively new occurrence and there's no evidence that it benefits health - quite to the contrary, viruses are implicated in the pathogenesis of autoimmune diseases such as ME/CFS or MS. [1]: https://www.bmj.com/content/349/bmj.g5267 https://www.bmj.com/content/349/bmj.g5267 [2]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5320962/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5320962/