Cheong-san is alive and a hambie
Probably the most discussed topic since the end of the show. There are plenty of pieces of evidence which support this; the fact that he used Gwi-nam as a shield from the flames and you Gwi-nam's body burns as the two fall but not Cheong-san's. The fact he had only had one symtpom of infection, which was shown by Lee Byeong-chan, who also became a hambie. Him outright telling Gwi-nam that he's not afraid of him. When the main characters reach safety and flashback to people they lost, only Cheong-san is not specifically shown dying.Gwi-nam is alive
Hambies are meant to be immortal as Word of God said. He can't be killed.Lee Byeong-chan is a hybrid
He seemed far more in control compared to the other zombies. His walk was human, almost robotic like compared to the other zombies.On-jo has full immunity to the virus
She's scratched in the school infirmary, and in the cafeteria, the first zombie that runs up to her seems to not even notice her — it sniffs around her, but doesn't actually attack. It seems like every time she's chased after that, she's with someone else.This show is a prequel to Train to Busan
Some years in the future, The Biotech company took Lee Byeong-Chan's virus formula and experimented on, than accidentally released the virus in natural water and the rest is history.