Stupor (From Latin stupere, "be stunned or amazed") is the lack of critical mental function and a level of consciousness wherein a sufferer is almost entirely unresponsive and only responds to base stimuli such as pain. Those in a stuporous state are rigid, mute and only appear to be conscious, as the eyes are open and follow surrounding objects. The word derives from the Latin  ("numbness, insensibility"). Being characterized by impairments to reactions to external stimuli, it usually appears in infectious diseases, complicated toxic states, severe hypothermia, mental illnesses (e.g. schizophrenia, severe clinical depression), epilepsy, vascular illnesses (e.g. hypertensive encephalopathy), shock (e.g. learning of a death or surviving a car crash), neoplasms (e.g. brain tumors), vitamin D deficiency and other maladies.