Chen-7

Chen-7, also known as the One Day Plague, could kill in a day. Only species with a binary vascular system, such as the Apalapucians and natives of Gallifrey, could contract it. In the case of Time Lords, regeneration would be impossible.

When a Chen-7 plague threatened Apalapucia, the Apalapucians created the Two Streams facility, which used temporal technology to isolate the patients and stretch out the day into a lifetime. (DW: The Girl Who Waited)