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).
In this case, the Aunt or Uncle is a sibling of the possible parent in question and the same rules as sibling testing apply. This type of test is best to do if the mother of the child c Aunt or Uncle is a full sibling to the possible parent as they will share more DNA.