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).
Stanley S. SquarePants is Sherm's son, the cousin of SpongeBob, Larry, BlackJack, and Todd, a nephew of Blue, Harold, and Margaret, a grandson of Grandpa and Grandma SquarePants, and a descendant of SpongeBuck, Primitive Sponge, and SpongeGar. In the future, he will be a 1st cousin 2x removed.