There are many different causes of fever. Infection may be present even when culture techniques do not reveal the source. For example, consider viral infections such as mononucleosis and hepatitis. Medications, vasculitis, malignancy, inflammatory bowel disease, central nervous system disease, hyperthyroidism, and other diseases can all cause fever. Your doctor can take a thorough history, perform an exam, and order appropriate tests to determine the cause.
Thank you for your question.

There are other causes of fever - reaction to medication, still’s disease, hyperthyroidism and many others. Generally, an internist or infectious diseases specialist can help.

G. Dickinson