Pretty often, actually, people get sick and then die. Especially people who've had cancer. :(
Reading between the lines here, I suspect his health is in really bad shape. I hope I'm wrong, though.
More to the point he has a disease that causes depression, fatigue, hair loss, urinary tract infections, and other embarassing conditions. So it makes perfect sense for him to stay at home to recover.
Other than he had Cancer 5 years ago what lines are you reading in between that makes you think he's sicker than he says?
Anything so serious that it could cause a CEO to leave a company for six months is extremely serious.
Second this isn't a significant amount of time off. Perfectly healthy 53 year olds with millions of dollars in the bank take sabbaticals all the time.
If you're that rich, you don't go to the hospital, the hospital comes to you.
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As far as "there’s always a reason" I have to ask: Have you looked at what the symptoms are for the disease he has? Fatigue, Hair Loss, Urinary Tract Infection, etc... I’d take a few months off if I were him. It doesn’t mean he’s any sicker than he says he is.