I was diagnosed with Chronic Eosinophilic Leukaemia (FIP1L1-PDG-FRA) in 2008. This disease is analogous to CML, very rare and is treated with Imatinib.
At the time, I was a Co-Pilot for Virgin Atlantic Airways. The UK CAA suspended my medical for about 8 months, by which time I have achieved CCyR and PCRU status.
I was reissued a UK CAA Class 1 flying Medical and resumed flight duties.
Since then, I have left Virgin in the financial crisis, joined a different airline, gained my Captain rank, become a management pilot and last year moved to one of the world's biggest mining companies as Captain on their corporate jet.
I fly the CEO worldwide and I consider this to be a vote of confidence of my wellbeing and longevity. He knows of my past medical history.
The UK CAA are only interested in what your Consultant says. They have no expertise at all in molecular diseases.
So, in short, there is absolutely nothing stopping anyone flying for a living as long as your illness is well controlled.
As for car racing and scuba diving - I can't imagine it being a problem at all.