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).
(3) great-grandparent, great-grandchild, aunt who is a sister of a parent of the individual, uncle who is a brother of a parent of the individual, nephew who is a child of a brother or sister of the individual, or niece who is a child of a brother or sister of the individual (relatives in the third degree).