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 are almost finished! You have only to look on the chart and intersect line 4 with column 6 to discover that the answer to the riddle is that the great grandmother's uncle's grandson is the asker's second cousin once removed.