The normal average temperature is 38.5°C, ranging between 38°C and 39°C.
You need a medical thermometer, electronic preferably rather than the old mercury ones, well-lubricated and gently slipped into the rectum for a sufficiently long time. Infections are often accompanied by fever (temperature above 39°C, accelerated breathing and heart-rate, and prostration). Illness may be free of fever and just as dangerous as an infection.