My dad is having frequent chills and high fever, what is the reason?

Long list: Prolonged chills and fevers is never normal. The lsit can include infections of virtually any body system, auto-immune diseases, reactions to medicaitons or diet, or ominous things like cancers. The list includes hundreds if not thousands of possibles, so it is essential that he start with a visit to doctor, a good history, a thorough physical exam, and then likely tests to sort out what is what.