How do people 'see' your invisible illness?

It's fair to say that most people I come across at work, and in my daily life, don't know I have a chronic condition.

Apart from the occasional day, when I'm limping because the pain is in my hips or knees or feet or ankles, and on those days I use a walking stick, I'm to be seen walking (albeit slowly) and shopping like any normal family man.

At work I really have to try to hide my condition for fear of losing my job, so I 'push through' whatever pain barrier I have to beat to do my job and, on the days when I fail to beat the barrier I have to call in sick - resulting in a risk of losing my job.

Thankfully my employers have 'recognised' my condition and are (for the time being) understanding to a degree - the same cannot be said for my immediate colleagues and lower management teams. Explaining that I'm not able to do certain tasks to my Team Leader or even Store Manager gets a 'blank' expression and the question "Why not?" to which I tend to respond "Well, my *back/elbows/feet/hands/neck/shoulder/leg/groin/arm/bowel is giving me jip." (*delete as appropriate) when what I really should tell them is  "I suffer from Fibromyalgia and Crohn's Disease - both are chronic conditions which do not improve and can only be treated with all manner of lotions and potions to enable me to function in a very basic way for most of the time. HR are aware of my condition and expect that, at least on some days, I cannot fulfill all of the roles required of my job function, on these days it has been agreed that I can only undertake 'light duties'"

But still they don't understand. Their role is to see that all of the tasks for the day are completed and having a member of the team who 'cannot play ball' just slows them down. But what if I suddenly announced that I was pregnant? Or I struggled in to work with a plaster cast on my arm - they'd thank me for turning up and give me light duties without question. However, 'light duties' generally means standing for my whole shift at the checkout - which causes no end of trouble for my back - but hey-ho, you take what you can get! At work my invisible illness is seen very much as an inconvenience.

Colleagues of the same role - who aren't on light duties - question Team Leaders as to why I'm not doing any of the heavy stuff and I have caught the Team Leader's response once or twice as "Oh, he's got a (finger quotes) bad back". Understandably this leads to a bit of resentment on the shop floor - I'm sorry (and ashamed) to say that I might have felt the same way not so very long ago.

Having an invisible illness is bad enough in itself, but to have to constantly explain to people what it is exactly that's wrong with you - even people you've told many times before - sort of gets you down, if not extremely angry. If you met one of your friends in the street one day and noticed that he'd lost a leg or an arm you'd remember next time you saw him, wouldn't you? And you certainly wouldn't go home and invite him to your next cycling club meeting.

So, how do you make an invisible illness visible? Wear a badge? Or tee shirt?

I'd love to hear how other sufferers explain their illness(es) to their colleagues, friends, even family. Hopefully someone will come up with an excellent phrase that I can steal!