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Chen-7, also known as the One Day Plague, was a pathogen capable of killing individuals within the space of a single day. Only species with binary vascular systems, including Apalapucians and Gallifreyans, could contract the disease. In the case of Time Lords, the Eleventh Doctor implied that it disabled their ability to regenerate.
When a Chen-7 plague threatened the planet of Apalapucia, the Apalapucians created the Two Streams Facility, which used temporal technology to isolate the patients and stretch out the day into a lifetime. (TV: The Girl Who Waited)