He died of cancer, but in fact died of exhaustion rather than because of the disease itself.
Because he was only 50 when he died people have sometimes asked why he wasn’t able to cure himself. What this question ignores is that in 1917 when the cancer was first diagnosed he was given just 3 months to live. From then until his death in 1936 he was curing himself, every day, for nineteen years – all the time it took for him to complete his work.