I had testicular cancer with 2 secondary tumours in abdominal lymph nodes. I can't say it affected my cycling. But why should it, it didn't hold Armstrong back. Actually there was a slight benefit in that occurrences of knackering the bollocks on the saddle were reduced by 50%.
I also have CFS/ME, for which cycling is really the only treatment which works for me. I have to build up very gently, otherwise I get a relapse. And I have to do it while touring in warm countries. It just doesn't work if I stay at home and go off on some dreary suburban loop every day with no destination except my front door. Perhaps the reason it doesn't work is depression. I get that very badly too. It makes it very difficult to find motivation. I've never liked cycling in the UK.
CFS patients are advised not to use more than 80% of their available energy on any given day. Otherwise you pay for it the next day. I haven't cycled at all for 3.5 years because I've had a series of relapses which turn me into a zombie. The zombie phase lasts for 3 or 4 months. I emerge to find a mountain of unopened mail, muscle wastage and a filthy flat. I spend 5 or 6 months gradually getting back to normal, then I try to do too much, or suffer some kind of external stress, and I have another relapse. So I'm trapped.
I reckon the only treatment which would work would be a year of touring in warm countries, starting very very gently and building up gradually. This is hard to afford when one's finances have been trashed by illness and the banking crisis. My psychiatrist suggested I should get funding from a charity...hmmm. Can't think of anyone to ask.
The stupid thing is that my benefits would more than cover indefinite cycletouring therapy. Then there's the savings to the NHS if I weren't around. But you can't claim benefits if you quit your abode and go and live abroad on the road. Imagine what the Daily Mail would say. It's a shame really. I reckon I could start contributing to the economy within 12 to 18 months if they would only let me decide what to spend my benefits on. The total is equivalent to a pre-tax salary of £32,000. It's crazy.