An undead is a being in mythology, legend or fiction that is deceased but behaves as if alive. A common example is a corpse re-animated by supernatural forces by the application of the deceased's own life force or that of another being (such as a demon). Undead may be incorporeal like ghosts, or corporeal like vampires and zombies. The undead are featured in the belief systems of most cultures, and appear in many works of fantasy and horror fiction.
Bram Stoker considered using the title The Un-Dead for his novel Dracula (1897), and use of the term in the novel is mostly responsible for the modern sense of the word. The word does appear in English before Stoker but with the more literal sense of "alive" or "not dead", for which citations can be found in the Oxford English Dictionary. Stoker's use of the term refers only to vampires, and the extension to other types of supernatural beings arose later. Most commonly, it is now taken to refer to supernatural beings which had at one time been alive and continue to display some aspects of life after death, but the usage is highly variable.
In 1932 Robert E. Cornish became interested in the idea that he could restore life to the dead. He attempted to revive human victims of heart attack, drowning, and electrocution but had no success. However, on test animals he managed to revive clinically dead dogs by injecting a mixture of epinephrine (adrenaline) and anticoagulants. Brain ischemia or cerebral hypoxia leads to poor oxygen supply and thus to the death of brain tissue. Some causes associated with brain hypoxia include drowning, strangling, choking, cardiac arrest, head trauma, and complications during general anesthesia. Treatment strategies for brain hypoxia vary depending on the original cause of injury and may include anticoagulation with Warfarin or Heparin and Epinephrine and atropine to get the heart to resume pumping. Hyperbaric oxygen therapy and brain hypothermia to delay cell death. Chemical brain preservation as a means of avoiding death and reaching the distant future is currently under research and development, whereas Cryonics is the low-temperature preservation of dead humans with the hope of resuscitation in the future.