A quarantine lockdown is a means to contain an outbreak of a disease, to prevent it reaching the general population. Atlantis has an automated system that detects and quarantines the infected, but also allows those who are able to help, to enter the quarantine area. This system was first encountered when an Ancient nanovirus was released from the Nanite lab. Unfortunately, the sensors in the area had been damaged by the recent flooding and the system was not activated until Dr. Peterson transported into the Mess hall. (SGA: "Hot Zone")