I think he just overestimated his health and how (Russian) prison affects it. Bad nutrition, no sports, bad conditions - some place may be too hot, some too cold, there may be various fungi, and simple illnesses which you can easily treat outside prison are much more dangerous, because prison medics do not really care if you survive.

And, of course, that all is without the administration trying to kill you by sleep deprivation, sun deprivation and whatever else.