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The Doctor Hunt was an operation that took place eight years prior to the start of the series, on Drum Island.
Being a king, Wapol saw himself as a powerful person so he decided to tighten his grip on Drum Kingdom. Since Drum was known for its advanced medical knowledge, Wapol decided to make people beg at his feet by taking away the doctors.
Wapol then sent out his army to capture any doctors they could find and forced them to live in Drum Castle. Any doctors who refused to be a subject under Wapol was either exiled from the country or killed. Doctors who gave in were called the Isshi-20. There were almost no doctors in the kingdom, anyone who was sick or dying had to beg at Wapol's feet. However, many Drum citizens refused to sell their souls to their king, and what little respect remained for Wapol vanished, along with any hope that he would be more like his father.
However there were two doctors left after the hunt who evaded Wapol's army, Doctors Hiriluk and Kureha. Hiriluk was not a real doctor and had no medical license and he was known to burst into a sick person's house and give the wrong type of medicine that would either kill the person or have them fall victim to a weird side effect, which earned him the nickname "Quack Doctor", although he genuinely cared for his patients as well as his country, and sought no reward other than their good health. His counterpart, Dr. Kureha was quite different in that she possesses boundless medical knowledge and experience (being a spry 139 years old) and can help assuage almost any ailment or injury. The only downside is that unlike Hiriluk, she seeks unreasonable reward for her services, but although it seems to be her only reason to heal her patients, she does show concern and a deeper wish to treat the doctor-stricken populace.
As time went on, Hiriluk and Kureha continued to defy Wapol's hold on the nation, so Wapol set a trap for the two remaining doctors: he falsely claimed that the Isshi-20 were ill, preparing to kill the two outlaws when they arrive to heal the suppressed doctors. Hiriluk, who was dying as he is, knew it was a trap as much as Kureha did, but he alone could not resist the temptation of the slightest chance that the Isshi-20 are truly ill, and went to Drum Castle alone. As expected, Wapol sprung out with his forces to kill Hiriluk, but the quack made his final speech on how he believed that the country will one day be truly healed, and then ended his own life by drinking one of his own botched experiments, causing his body to explode. While Wapol, Chess and Kuromarimo laughed with no respect, Dalton and the Isshi-20 saw Hiriluk's willingness to walk into a trap for the good health of his enemies as a noble sacrifice, and saw him as a role model and inspiration to rebel against the tyrant.
Years later, the Straw Hat Pirates finally defeated and banished Wapol from the country for good, thus allowing the doctors to return to the kingdom. The Isshi-20, who were perfecting their own skills while working under Wapol for the betterment of the people, were forgiven and allowed to return due to them hating their former king as much as the others, and they eventually grew out to become the Isshi-100, who are 100 doctors working under Kureha.