I'd read he had Non-Hodgkin lymphoma and as a result production of the second series was delayed and he was then declared cancer free but it came back so he had to be replaced for the leading role.
Amusing anecdote: One of Kirk Douglas's sons is / was a stand up comedian and played the Comedy Store in London. He wasn't going down very well, and threw a bit of a tantrum declaring : Don't you know who I am? I'm Kirk Douglas's son, to which someone at the back stood up and shouted "No! I'M Kirk Douglas's son..."