"High intelligence" here didn't mean that the regressed players could think and fight like normal people. Rather, their fighting instincts surpassed those of ordinary Mutant Animals. They weren't normal people, and they couldn't respond to what people said, which let them avoid the effects of Abilities and items to some extent. Years of life-and-death struggles in game Dungeons had also honed their instinct to fight efficiently in a crisis. So, with the country and government completely unprepared, District 010 lost a large number of Players. Players attacked by them also had a chance of regressing.
The toxins from Mutant Animals seemed to target Players specifically. In cases of direct contact, Antidotes had no effect at all. Also, Alien who had regressed from Players were more organized than ordinary people and more likely to obey orders. They quickly chose a leader by instinct, then worked together to hunt. Some group-living animals could do this too, but regressed Players seemed more efficient, as if they still possessed the intelligence of "people."
This wave of regression, which broke out among Players, dealt District 010 a devastating blow. It was around then that Players began leaving their Sub-districts one after another. A virus that could turn Players into beasts in an incredibly short time meant that even their homes weren't safe. With the harsh external environment and Black Thorn Disease spreading once again, the Player community began to realize that District 010 was sliding toward the abyss, and there was nothing they could do to stop it. To protect themselves and their families, some decided to cut their losses.
Once Players began to retreat, the first line of defense was already on the verge of collapse. Alien hordes that had been breeding somewhere came surging back, soon destroying countless cities. The Survivors in those