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).
For other uses, see Nephew (disambiguation). A nephew is a son of a person's sibling, and a niece is a daughter of a person's sibling. Conversely, that person is the aunt or uncle of their niece or nephew. The gender-neutral term encompassing both nephews and nieces is nibling.