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A fever is an elevation of the body temperature significantly above the normal range. Notice I said significantly - "Doctor, she has a fever of 99º rectally!" "Oh my goodness!"
Normal rectal temperature in a young child is about 99-100ºF rectally. (See temperature) With newborns and infants, I usually consider up to 100.4ºF rectally probably normal; 100.5-101ºF possibly a fever; and 101ºF or greater definitely a fever.
Fever is caused by disease, almost always infection. Teething does not cause fever. And fever is harmless. Any temperature shy of 107º (which you will never see) is absolutely harmless to the child. Repeat: fever does not damage the brain or do anything else except make the patient feel bad and help the immune system fight infection. That's it.
Fever can be treated or not, depending upon the parents' judgement of how uncomfortable the child is with his fever. Generally, pediatricians don't recommend treating fevers under 102ºF, but in my experience parents get pretty tense at 101ºF so I don't argue with them about it. Besides, treating 101.5º at 8:30 p.m. is easier on all of us than treating the 104.5º at 11:30 p.m.
A fever is like a wave on the ocean - it goes up and down and up and down. Parents rejoice when it goes down, and despair when it goes up. But this is all part of the natural cycle of fever.
Fever is caused by the release in the body of certain chemicals called pyrogens from white blood cells as they attack foreign invaders (germs). These pyrogens travel via the blood stream to the temperature regulating center of the brain. The "thermostat" in the brain is reset to a higher temperature, rather like resetting the furnace thermostat in your hallway at home. The brain then coordinates various bodily responses to raise the body temperature until the new thermostat setting is achieved.
The temperature rises at first, often rapidly. The child feels cool to the touch as his body tries to conserve heat by constricting the blood vessels in the skin. She may shiver as she tries to generate heat by muscle activity. This early part of the temperature rise seems to be the most vulnerable time for convulsions (febrile seizure) if they are going to happen. Unfortunately, the seizure has almost always happened before the fever medicine has kicked in, and often before the parents even knew anything was wrong - the skin felt cool.
Then a plateau phase is reached. The new thermostat setting has been reached. Now the skin feels very warm to the touch because the blood vessels there dilate to help radiate the escessive heat being generated. The child will throw off the covers.
Then the pyrogen dissipates in the bloodstream, the thermostat in the brain returns to its normal setting, and the body temperature starts to fall back towards normal. Now the child may sweat to help get the elevated temperature back down to the now normal thermostat setting. Parents breathe a sigh of relief.
A new wave of pyrogen release starts the whole cycle over again.
Why do humans have fever? Why do children have so many fevers, and why so high?
Humans have the fever response to infection for the same reason as all other creatures from reptiles up to mammals. Yes, even cold-blooded reptiles exhibit a "fever" response to experimentally induced infection. Experimental animals have a higher survival rate if allowed to sun themselves and raise their body temperature, than if they are kept in the shade and cold. In some way, elevation of the body temperature aids the immune system in fighting infections. That's basically the rationale for not treating fever - nature put it there for a purpose - why interfere?
Children have so many more fevers than adults because they are born without immunity to the zillions of different disease-causing germs floating around in their world. The immune system fights off the inevitable infections and becomes immune to more and more common germs over time. Eventually, the child's naturally acquired immunity will prevent most germs from entering the body and causing disease.
Children do seem to run more high fevers than adults but I'm not absolutely sure why that is. It is possibly based on the fact that they are smaller in body mass - smaller mammals tend to run higher normal temperatures. More likely, I would think, is that the viral illnesses that seem to cause the highest fevers are generally acquired in childhood rather than adulthood (by which time most grownups are immune).
The treatment of fever includes a couple of options:
No aspirin for fever or anything else in childhood unless directed by a specialist for such things as heart or arthritic conditions.
And please don't forget that when anyone has a fever, the heart rate and breathing rate speed up to help the body get rid of excess heat - just like a dog panting in the summer.