What was Shackleton's illness?

I've only seen the show once when it first came out, when is this illness the OP asked about mentioned or shown? I can certainly imagine Shackleton suffered from a run-down body after all the physical and mental strains he endured on all those Antarctic expeditions, from the Scott expedition onward - so a bad back is certainly very plausible! He of course died very young at age 48 from a heart attack during his final Quest expedition in 1922.