Fever can support the immune system's attempt to gain advantage over infectious agents, such as viruses and bacteria, and it makes the body less favorable as a host for replicating viruses and bacteria, which are temperature sensitive. Infectious agents are not the only causes of fever, however.
Your sister's grandson is your great nephew. You are his great aunt (if you are a woman) or his great uncle (if you are a man).
My Cousin Rachel was published in 1951, and is a gripping story of suspense. It very much recalls the feel and style of her earlier mystery novel Rebecca of 1938. The novel's narrator is Philip Ashley, a young man who had been orphaned at the early age of seven.