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).
AUNT: This is a female relative. She is the sister of your parental ancestors (father, mother, grandparents, etc.). Some children refer to close female friends (of their parents) as an Aunt, all though there is not any true familial relationship. You would be her (your aunts) niece or nephew.