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).
We are speaking here of cousins and removes as in 4th cousin twice removed. Everyone seems to know, even instinctively, that parents of a parent are their grandparents and the sibling of a parent is an uncle or aunt.