Chen-7

Chen-7 (also known as the One Day Plague) was a deadly plague that could kill in one day and was only contractable by species with a binary vascular system, such as the Apalapucians and natives of Gallifrey.

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)