Fever is a frequent symptom which can arise as a consequence of bacterial, viral and parasitic infections. Additionally, fever is a frequent symptom in non-infectious diseases, e.g. autoimmune diseases or lymphomas. The term dangerous fever describes febrile conditions which are associated with high mortality and, in most cases, mandate hospitalization, and even monitoring in an intensive care unit. As a rule, infections (bacterial infections in particular) may begin harmlessly; however, they can instantaneously change into life-threatening conditions even a few days later. In the following report we focus on some critical syndromes which must be recognized quickly, such as severe sepsis, septic shock, fever in neutropenia, and fever in the immunocompromised patient.