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Chen-7, also known as the One Day Plague, was capable of killing individuals within the timeframe of a single day. Only species with a binary vascular system, namely Apalapucians and Gallifreyans, were vulnerable to the disease. In the case of Time Lords, regeneration would be impossible.
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. (DW: The Girl Who Waited)