Fever also known as pyrexia is caused when there is an increase in our body temperature. It is a sensation of some infectious disease that is weakening our immune system. This fever can be normal or high. Fever can be tested by The use of thermometer . 
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an abnormally high body temperature, usually accompanied by shivering, headache, and in severe instances

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Fever is departure from normal health due to structural or functional disorder of the body. Fever is caused mostly due to diseases. For example:- Common cold, mumps, etc. are caused due to viruses.