Abnormal rise in body temperature above 38ºC as a consequence of an illness. Fever occurs because of an imbalance between heat elimination and production. Exercise, anxiety, and dehydration may increase temperature in healthy individuals. Infection, neurological diseases, malignancies (tumor fever), anemia (neutropenic fever), as well as many drugs can cause fever. Fever has no known function except in infection. In other cases is a warning call that must be always considered. It is associated normally with chills, headache, tachycardia, tachypnea, and general malaise. In the case of hematological disease, it should be reported to the physician without delay.