Just wanted to show my support for this. DH is in this position, has been his whole life, sometimes people make unfair judgement about him as a result. He has a rare condition, of which he falls into rarer sub-sets, so even those who know about the illness will sometimes say, "Oh, so-and-so has that and is fine". Unfortunately for my DH his illness is very much active! On the outside he looks problem-free, but I'm the one who sees him at home trying to manage it, and struggling with the effects. He gets embarrassed and is very private and will tell everyone he is fine too, as understandably he doesn't want to detail to all and sundry his medical state especially when he has been dealing with this since childhood. One of the registrars at the hospital called him a veteran! Such early onset is one of the very rare sub-sets that he falls into.
He gets odd looks if he pulls out his Radar key in public to use the disabled toilet when another is not available. I don't know what they think he must have done to get it, it's not like they're hard to get hold of!
Additionaly I have a chronic illness, it is not a disability and it is much more easily managed than conditions like DH's. But it still impacts me, and people don't quite 'get' it. Part of it means I get tired easily and have musculoskeletal problems. According to many, a lack of exercise is my problem
Notwithstanding the fact that I am active, if exercise was the cure I'm sure the NHS would be much happier recommending that than handing out free daily medication for life! (I have a medical exemption certificate as a result of my condition)
I don't expect people to know that you have an unseen disability, or to regularly think it's possible, especially if you're like my DH and never talk about it with others. But what I dislike is the way people make judgements, like if you've been off sick for a long time, or if you go to the toilet regularly, or have regular breaks, or often look tired, for example. I find that too often people class you a 'skiver', or lazy, or similar, without even beginning to entertain the thought that you might have good reason, because you "look fine" to them.