How does physical illness relate to madness in Heart of Darkness? How does one’s environment relate to one’s mental state in this book?
1) The most obvious connection between madness and physical illness can be found in the character of Kurtz, who suffers from an unnamed tropical fever and succumbs to it at the end of the novella. When Marlow encounters Kurtz, he is emaciated and reduced to a hollow shell of his former self. He is haunted by his past, infatuated with collecting ivory and brutalizes the Natives. After Kurtz’s death, Marlow is given the dead man’s official report for the International Society for the Suppression of Savage Customs and is impressed by its eloquence and precision. At the end of the writing though, a postscript is scrawled reading “Exterminate all the Brutes!” This small detail is just one of several examples tracing Kurtz’s mysterious descent into savagery. Whether his physical illness or mental illness came first is up for debate, which brings us to your second question.
2) Your second question gets to the heart of a primary theme of the novella. One of Marlow’s primary obsessions is with the enigmatic relationship between the environment of the Congo and the evils he witnessed there. To Marlow, it seems that the darkness of the environment itself is a source of brutality and evil. The jungle is described as “the heart of impenetrable darkness” and “malevolent” among many other negative descriptions. The implication of this imagery is that, for a European at least, to spend time in the dark jungles of the Congo is to risk losing their sanity and even humanity.
Kurtz, once again, is a primary example of the negative impacts of one's environment can have, even on promising men. Kurtz is described by many as a Renaissance man with a bright future in politics and business. He was a painter, a poet, and a philosopher who “enlarged the mind” of all he met. However, with just a few months of isolation in the jungle, Kurtz has transformed into a ruthless monster who decorates the boundaries of his cabin with decapitated heads. The environment in Heart of Darkness plays a major role in the madness experienced by many of the characters in the novel, and this theme is best embodied in the character of Kurtz.
Physical illness is frequently invoked in Heart of Darkness, sometimes in direct relation to mental illness and what we could call moral illness—the depravity or metaphorical darkness that has entered Kurtz's soul.
The story of Kurtz and his descent into madness and subsequent death is told by Marlow, who succumbs to illness himself before leaving the camp.
While not explicit about the kind of illness, the main context for Kurtz's illness and related terrifying behavior comes from the Russian's account (as Marlow recalls it).
He had, as he informed me proudly, managed to nurse Kurtz through two illnesses (he alluded to it as you would to some risky feat), but as a rule Kurtz wandered alone, far in the depths of the forest.
These wanderings, lasting many days, might have been a cause or a symptom of his illness. In either case, while trying to obtain ivory, Kurtz raided the countryside, apparently shooting Africans if they refused. The Russian says
they had never seen anything like it—and very terrible. He could be very terrible.... [T]here was nothing on earth to prevent him killing whom he jolly well pleased.
When Kurtz threatened to shoot him, the Russian gave him some ivory he had gotten by trade.
But I didn’t clear out. No, no. I couldn’t leave him....He had his second illness then.
The Russian interprets these behaviors as suffering, and describes Kurtz as if he were lost, saying that he would
disappear for weeks; forget himself amongst these people— forget himself—you know.’
When Marlow sees this as a description of lunacy, the devoted Russian (possibly not too stable himself) defends him.
"Why! he’s mad," I said. He protested indignantly. Mr. Kurtz couldn’t be mad. If I had heard him talk, only two days ago, I wouldn’t dare hint at such a thing....
With these actions and afflictions s so closely tied to Kurtz's wanderings in the jungle and the related mentions of darkness, Conrad keeps the interpretation ambiguous; perhaps Kurtz's own realization of the horrors he has committed is what does him in.
Are you asking whether nature or physical illness contributed to the madness? Some physical illnesses can lead to madness. In Conrad's day, they were often conditions that are easily treated now, such as syphilis. As for environment, there seems to be something about the jungle that induces madness. My father was in Vietnam, and although he would never read Heart of Darkness he described something very similar. I do not think the jungle alone leads to hallucinations though. Being in an unfamiliar place, and being scared, can lead to breaks with reality.
Remember too that the madness in this story is symbolic. Africa was the last great frontier, and Marlowe is Conrad’s commentary of the madness of imperialism. Africa was not meant to be tamed, and is dangerous to those who try.