What Causes Fever Every Evening?

My husband is sick, his temperature is normal in the morning and usually rises over 100 F in the afternoon. We thought it was the reaction to the medicine levofloxacin because it is about 4 hours after the medicine. We stop the medicine today(not took it at 12pm), his temperature still rise up to 100.5F. It has been like this for 6 days? can you tell me why or what? thanks, Mina