From what I believe, Marlow fell sick due to being exposed to the true darkness which lurks in everyone, depicted through Kurtz's final line the horror the horror! The Russian describes that Kurtz opened his eyes and "enlarged his mind " which in Marlow's case, it opened his eyes to the true evil and "darkness " of humanity. Kurtz is the image, the warning of what will be coming to Marlow if he continues to stay in the Congo when Kurtz tells Marlow to "go away" " hide yourself ". The vague statement doesn't clearly define what Kurtz wants Marlow to hide from so as the audience we must define what he is told to hide from. We see as Marlow is being lured further and further into the depths of the dark forest of the Congo when he states, "And I remember I confounded the beat of the drum with the beating of my heart " representing his lack of ability and judgement to differentiate between the Congo and civilisation as he is lured further into the forest. But fortunately, Marlow comes to a realization that he can no longer live in the forest as he has been exposed to the "horrors " of the forest and ultimately escapes before it was too late.
After they return to England, Marlow meets Kurtz's intended and when asked about what his last words are, Marlow lies and tells her that "The last word he pronounced was " your name " when in reality, Kurtz said his infamous line "The horror, the horror ". The intended can be seen as the representation of the European civilisation as the intended was described with the use of light connotations, "fair hair.pale visage surrounded by an ashy halo " where the European civilisation is also represented through the light imagery. Marlow lies to Kurtz's intended about his last words as a way of protecting her from the darkness of the world. Marlow believes that women are pure and protects them from the reality of humanity and its darkness.
