Severe acute respiratory syndrome (SARS) has its greatest effect on the human respiratory system but also may affect other body systems. The virus usually enters the nose and quickly invades cells and begins to replicate. Infection then progresses and may spread. Symptoms often are at first mild but may become severe. Some people suffer such severe respiratory distress that they cannot breathe on their own. SARS also may directly damage organs of other body systems, including the liver.