The condition Irene and her friends suffer from and which the Iso Rings are designed to protect against appears to be primary immunodeficiency, which is a real condition: a weakness [usually genetic] in the immune system that leaves a person much more vulnerable to viral and bacterial infections.
On the commentary inside "Batman Beyond: Meet the Series Creators" found on disc two of Batman Beyond-Season One (DVD), Alan Burnett admits he thought up the Repeller when he was only 10 years old.