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 child of your great aunt is a cousin but is once removed because you do... In genealogical terms, this person is your first cousin once removed. Your mother's mother is your grandmother. Your mother's mother's sister is your great aunt, sometimes called a grand aunt. This person is also your mother's aunt. The child of your great aunt is a cousin but is once removed because you do not belong to the same generation.