Story related achievement is called OUTBREAK OUTBREAK
During chapter 4 Bad cop You will enter a room after doing a laser door puzzle you will see one of the pods that freeman used to get to Breen's office at the citadel in half life 2 you need to hit a button to move the pod then enter the area where the pod used to be a camera will appear and shot you with the CFLU virus all you need to doo is walk straight ahead to the vaccination station and the achievement will unlock.