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 father of your mother or father. But note the inconsistent use of grand and great. The brother and sister of your grandfather is your great uncle and great aunt (vide infra, Great Uncle, Grand Nephew). Presumably the word stems from the French grand-pÃ¨re, which itself goes back to the 12th century.