A fever is an elevation of the body temperature significantly above the normal range.
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.