今度はlice

　⇒A Lousy Reason for Asthma and Allergies | Popular Science

Since parasites like Polyplax serrata dampen innate immune responses, they help protect against immune dysfunctions like asthma or allergies. According to the study, populations without immunosuppressive parasites may have "unnaturally" elevated levels of innate immune activations, and therefore increased risk of immunopathology.

　まあ、日本的にはよく聞くお話。