Apocalyptic and post-apocalyptic fiction is a subgenre of science fiction, science fantasy or horror fiction literature in which the Earth's technological civilization is collapsing or has collapsed. The apocalypse event may be climatic, such as runaway climate change; natural, such as an impact event; man-made, such as nuclear warfare; medical, such as a plague or virus, whether natural or man-made; or imaginative, such as zombie apocalypse or alien invasion. The story may involve attempts to prevent an apocalypse event, deal with the impact and consequences of the event itself, or it may be post-apocalyptic, set after the event. The time frame may be immediately after the catastrophe, focusing on the travails or psychology of survivors, the way to maintain the human race alive and together as one, or considerably later, often including the theme that the existence of pre-catastrophe civilization has been forgotten (or mythologized). Post-apocalyptic stories often take place in a non-technological future world, or a world where only scattered elements of society and technology remain.