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).
That uncle has a brother who has a son. That son is your counsin's cousin, and your first cousin once removed. Three times removed is your cousin's cousin's cousin's cousin. Not exactly a close relative. This is not to be confused with a second cousin. Your second cousin is your mother or father's first counsin.