As I see it, the main two problems with many disabilities are, 1. They are invisible, and 2. If you haven't experienced it, you can't know what it's like.
Sciatica is a prime example as Colin mentions, most excruciatingly painful and unremitting, but when you're young, you hear old people saying they've got a 'bit of sciatica' and it means nothing, but it breaks through your sleep and haunts your days. However much someone had told me that years ago, I wouldn't have been any the wiser!
Epilepsy is another. I have three sons, two of whom have grande mal epilepsy of various types. My oldest son was asked at a job interview how he had caught it! That son loves pike-fishing, sitting alone for days on a remote river bank. I used to worry when he was young, and try to stop him going, until I realised that he needed the seclusion to relax, and although he was more likely than the rest of us to get into difficulties, the alternative was a loss of his freedom!
He has now 'grown out of it' to the point where he is un-medicated.
My youngest son still suffers major seizures, but is an assistant manager in a department store, who just happens occasionally to have fits, and very rarely, altered states where he can wander off and behave uncharacteristically, laughing and swearing, but not knowing where he is. Fortunately, he is very good at his job, and his employers understand the problems.
I suffer from agorophobia, and panic attacks, which can be quite disabling at times, especially as no-one else knows what is going on.
And it gets boring having to explain to people all the time!
The main thing I try to keep in my head is that I can't see what anyone else is dealing with, so always make allowances on that basis, because you never know!
And people will always find something to laugh at me about, so I laugh first!