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).
Your second cousin is the grandchild of your great-aunt or great-uncle. You share one set of great-grandparents with your second cousin, but you do not have the same grandparents. Third, Fourth, and Fifth Cousins. Your third cousin is the great-grandchild of your great-great-aunt or great-great-uncle.