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).
Q: Siblings? A: Well, technically William is the son of Kate's first cousin, as Kate and Diana's fathers were brothers, first and second in line for the throne. But I think of them as brother and sister, because they couldn't be any closer genetically without being clones, frankly.