Jack was a researcher for Yates, and he was the one who discovered the G-virus, a derivative of the X-virus that might cure those infected. But he knew Yates would just sell it at a high price, so he stole it and took it home.
But the Yates company has eyes everywhere, and when they found out, they sent mercenaries to take the virus. Jack's only chance to protect it was to inject it in his own body.
When the virus interacted with his cells, they changed him completely. Jack was no longer human, he was something else. He was strong and almost invincible. But the G-virus drove him mad and erased his consciousness. Jack has become a cruel, violent and irrational monster.
But the Yates company has eyes everywhere, and when they found out, they sent mercenaries to take the virus. Jack's only chance to protect it was to inject it in his own body.
When the virus interacted with his cells, they changed him completely. Jack was no longer human, he was something else. He was strong and almost invincible. But the G-virus drove him mad and erased his consciousness. Jack has become a cruel, violent and irrational monster.