Issues of time also enter into the fever equation. Some diseases cause cycles of temperature elevation, and one measurement as you arrive at the doctor's office or emergency department may have little relation to what your temperature is at other times. Studies of temperature during emergency department visits have shown that some patients with infections come in without a fever, but develop one during a several hour visit. Causes of fever vary. We worry about all of them.
Infections cause fever through the body's response to chemicals released by germs.
Some drugs, most notably some used for anesthesia and for control of emotions, can reduce the body's ability to regulate temperature and cause a fever. Hard to say. Most doctors would probably agree that an accurately measured temperature several degrees above normal should be investigated. Medications, infections, and metabolic causes should be considered. Some believe that a slight fever is beneficial when you have an infection, the theory being that germs can't survive as well in the hotter environment.