It's normal for your body temperature to fluctuate throughout the day, and you'd also consider it natural to run a fever if you've come down with the flu. Having a fever means your body is fighting an infection of some sort or another, from the mundane to the severe. On one hand, it could be a symptom of a cold or flu, while on the other hand it could be an early symptom of cancer. A fever may or may not need to be evaluated by a doctor, and even those that rise a little high may not be life-threatening emergencies — but how do you know?
OK, let's first look at the baseline. Normally a body's thermostat is set to 98.6 degrees Fahrenheit (37 degrees Celsius), and an elevated temperature isn't considered a fever until it rises above 100.4 degrees Fahrenheit (38 degrees Celsius) [source: ACEP]. Most fevers won't last longer than 24 to 48 hours, and most can be treated at home with acetaminophen or ibuprofen.
Fevers that rise above 103 degrees Fahrenheit (39.4 degrees Celsius) or that linger longer than 72 hours should be evaluated by your doctor. You may require antibiotics or at the very least additional testing to tease out the fever's source. Fevers registering 104 degrees Fahrenheit (40 degrees Celsius) or higher should be considered an emergency. Most fevers won't rise above 105 degrees Fahrenheit (40.5 degrees Celsius), but brain damage is known to occur when the body's temperature climbs above 107.6 degrees Fahrenheit (42 degrees Celsius) [source: Kaneshiro et al.].
Additionally, fevers sometimes occur alongside symptoms such as chest pain or difficulty breathing — two potentially life-threatening conditions that need immediate evaluation. And sometimes high fevers may cause convulsions, called febrile seizures; this is most common in children between the ages of 6 months and 5 years, with a fever of at least 102 degrees Fahrenheit (38.8 degrees Celsius). Anyone who has a seizure should be examined by a doctor, and any seizure lasting longer than 10 minutes needs immediate attention [source: NINDS].