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).
You and the children of your parents' cousins are second cousins and share at least one great-grandparent. Your child and your cousin's child are second cousins. Third cousins. You and the children of your parents' second cousins are third cousins and share at least one great-great-grandparent. And so on with the fourth, fifth, and sixth cousins. First cousin once removed. A relationship that is removed is one that exists in two different genealogical generations.