On the night of Tuesday 29 September, Cortini responded to a disturbance at the Mizoil Gas Station in the plains. The disturbance was caused by a Zombie, but he was unfamiliar with the mutants and tried to detain him. During the scuffle, he was distracted by the arrival of a customer, either Leon S. Kennedy or Claire Redfield, and had his throat torn. Despite his injuries, Cortini survived long enough to mutate into a Zombie.
The Ghost Survivors
In the "non-canon" scenario in Ghost Survivors, Cortini is able to shoot the zombie. Before he is able to leave the shop, he realizes that he is now surrounded by various creatures and cannot escape. With no way out, he holds out for as long as he can against the impending zombie horde. Miraculously, Leon, who arrived to investigate as well, saves Cortini. The two then leave the gas station heading towards Raccoon City.