In Western meieval legend, an incubus (plural incubi) is a demon in male form supposed to lie upon sleepers, especially women, in order to have sexual intercourse with them. It was believed to do this in order to spawn other incubi. The incubus drains energy from the woman on whom it performs sexual intercourse in order to sustain itself, and some sources indicate that it may be identified by its unnaturally cold penis. Religious tradition holds that repeated intercourse with such a spirit by either males or females (the female version of the incubus is called a succubus) may result in the deterioration of health, or even death.