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.