My brother had encephalitis as a teen, which resulted in debilitating depression, immune system and neurological issues that persist to a slight degree even 16+ years later. When he was just returning to a somewhat "normal" life and attending college, he confided in a great deal of frustration with this sort of issue. My suggestion was this response, which he got a kick out of and reportedly worked great at getting the busybodies to back off.
"So, what EXACTLY is wrong with you."
To all of you who are dealing with non-obvious or intermittent disabilities, I raise a glass to you; I know from my brother and another relative who suffered a TBI that left lasting issues that the comments and behavior from "outsiders" can be worse than the disabilities themselves in some cases.