Immortality (eternal life) is the idea of something which is alive that will stay alive forever. The opposite of immortality is mortality, which means a living thing can die. Right now, only a small number of living things are known to be biologically immortal. These are mainly simple, lower forms of life like bacteria. Although they are still mortal as they can die from other causes. Stories about immortal people or animals are popular in fiction and mythology, but not yet possible due to the decline of health as the years pass.