Fever is a common clinical phenomenon. In most cases, the cause of fever can be determined quickly. The clinical problem starts when the patient has fever for a long period without any easily detectable cause. The cause of fever can be classified as follows: infectious agent, neoplasm, immune or metabolic disorder, genetic abnormality of inflammasome functioning. The clinical course of fever may vary (continuous, remittent, intermittent, hectic). Detection of the cause of chronic fever is not always possible. In these cases fever of unknown origin is diagnosed.