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).
Children of your parents' brothers and sisters. You and your first cousins share one set of grandparents. Double first cousins. If a pair of brothers marries a pair of sisters, their kids are not only first cousins, they're double first cousins: They have both sets of grandparents in common. Second cousins. You and the children of your parents' cousins are second cousins and share at least one great-grandparent.