Fever is caused by disease, almost always infection. Teething does not cause fever. And fever is harmless. Fever can be treated or not, depending upon the parents' judgement of how uncomfortable the child is with his fever. 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. These pyrogens travel via the blood stream to the temperature regulating center of the brain. 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.