In Heart of Darkness, why does Kurtz get sick?
We don't really know what causes his physical illness; we can assume that he has caught malaria or is suffering from some other fever illness.
However, he has looked into the depth of his soul (the heart of darkness) and what he has seen there and discovered within himself is corruption and depravity.
Marlowe recognises that not only does Kurtz suffer because he has seen into the depths of his own soul and found corruption but he recognised that all men have this possibility in within themselves. This 'seeing' has made Kurtz ill.