In healthy people, procalcitonin (PCT) is produced in thyroid cells as a precursor for the hormone calcitonin and cannot be found in human blood. However, bacterial infections cause PCT to be produced by almost every organ of the body, resulting in a rapid rise of PCT levels in the blood. Viral infections do not cause an increase in PCT blood levels. The level of PCT in the blood is a reflection of the severity of bacterial infection, ranging from slightly elevated concentrations in infections with a minor systemic inflammatory response to very high values in cases of severe sepsis and septic shock. Once an infection is under control, PCT levels decrease rapidly. This makes PCT an effective tool which aids physicians in the diagnosis of bacterial infection and sepsis.