I feel like in order to understand what is normal, one must have a small understanding of what is abnormal. The "fever" in this novel was the yellow fever. Symptoms were yellow eyes, and yellowish tint to the skin of the white people in the novel. This is partly why they thought the black people couldn't get yellow fever. So, to answer your question, you had normal health if you didn't have the symptoms of a normal fever and your eyes and/or skin were their normal color.
A) rare to find and hard to form because in the passage, it mentions that you need specific conditions but they are hard to find (because they get destroyed before they are discovered)
