Stupor is the lack of critical cognitive function and a level of consciousness wherein a sufferer is almost entirely unresponsive and only responds to base stimuli such as pain. A person is also rigid and mute and only appears to be conscious as the eyes are open and follow surrounding objects. The word derives from the Latin stupor, meaning insensible. Being characterised by impairments of reactions to external stimuli, it usually appears in infectious diseases, complicated toxic states, severe hypothermia, mental illnesses (e.g. schizophrenia, severe clinical depression), vascular illnesses (e.g. hypertensive encephalopathy), shock (e.g. learning of a death or surviving a car accident), neoplasms (e.g. brain tumors), vitamin D deficiency and so on.