No Time to Talk
Get to the lab as fast as you can
You'll hear a mysterious Voice, which orders you to go to the lab as fast as possible. You'll be moved automatically to the interior of the island. To get to the lab quickly (3) use Fast Travel option (2) #1. Unfortunately something horrible happened and everyone is infected #2.
Go to the room where you met Dr West - you'll see several undead trying to get inside #1. Kill them and enter the room. Use a computer #2 to free Yerema locked in a cell.
She will tell you what happened in the facility #1. Nervous The Voice will propose you to search for a vaccine - maybe Dr West managed to create it. Antidote can be found in a small fridge in the same room #2.
Act 3 ends at that moment. In order to move to the next Act and go to the prison, answer the CB radio call.
WARNING: After you go to the prison you won't be able to get back, so think twice before you continue with the main plot.