The Sherman's Plague virus became dormant circa 600 BCE, but was reintroduced into the planet's biosphere in 2376, when resident Abraham Auerbach was excavating his land for the installation of a swimming pool. Auerbach became "Patient Zero", initially suffering cold-like symptoms, then later chest pains, stomach cramps, coughing and vomiting blood before dying of massive organ failure.
The disease spread quickly around the planet, due in part to transporter transportation and the inability of their biofilters to identify the virus. Within days, 70 percent of the population was exhibiting early symptoms, and the world's medical community was particularly hard hit. Because of this, Doctor Elizabeth Lense, chief medical officer of the visiting USS da Vinci took the lead in developing a cure.
Lense ultimately devised a method in which a second, artificial virus would be released into the atmosphere, which would implant an extra chromosome into each victim's DNA, enabling their immune systems to combat the Plague and repair the damage it caused. Though technically a genetic augmentation, Captain David Gold argued that Human inability to survive the native conditions of their home world constituted a birth defect, allowing Lense to legally use her method.
Ultimately, the disease was stopped, with fatalities limited to on half of one percent of the population. Two samples of the pathogen were preserved afterward, with one left at Sherman's Planet's main medical facility, and one kept by Lense, presumably for study at Starfleet Medical. (SCE eBook: Oaths)