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).
The children of your nieces/nephews (your siblings grandchildren) are your grandnieces/nephews, and their children are your great-grandnieces/nephews. Grand-niecs/nephews are also sometimes called great-nieces/nephews, and great-grandnieces/nephews are also called great-great-nieces/nephews.