"Soul flu" was a term Spike used to refer to a spiritual parasite that he was afflicted with during the James crisis. After he was revived as a ghost preceding the Battle at the Hellmouth, Spike was infected by the parasite, though it lay dormant in his body for years until the temporal fold resulting from the Fall of Los Angeles awakened it, corrupting Spike's soul and influencing him to perform some morally ambiguous actions such as seducing the vampire Felicia Valentine. However, he remained unaware of the parasite's presence until the Soul Eater Liss Hubble claimed him to be soulless.