Over 99 degrees. The body has a fairly tightly regulated normal temperature. Elevated temp is in response mostly to infection as the invading organisms have adapted to 98.6 and our defense mechanisms work the best at elevated temp. Depending upon the source of the fever, you can tough it out but if you are uncertain, you might need to see your family doctor.
Depends on time. Human body temperature is a diurnal phenomenon, with the lowest temps occurring in the early am and highest (less than 100) in the late afternoon or early evening. 98.6 is actually a midrange average and 99 degrees is seldom representative of fever.