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).
Amelia Fieldmouse is Mickey's older sister and the mother of Mickey's twin nephews Morty and Ferdie. The character first appeared in Morty and Ferdie's 1932 comics debut. In the Dutch translation of the strips, her name is given as Amalia.