Abby, having bitten off Ellie's fingers and ingested her blood falls sick when she gets to the island and goes into a coma.
When she awakens she finds the island is now infested -- The islands occupants have been doing experiments on her when she was unconscious, knowing that she is now immune.
Unfortunately instead of a cure this led to the birth of a new variant of infected, with much higher intelligence, which now overrun the island.
Abby escapes the island and goes on a quest to find Ellie who has gone into hiding.