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Building 27, built in 2004, is the Astronaut Quarantine Facility. Prior to going into space, all the crews were placed in quarantine a week before they launched to protect them from viruses, colds, and germs that they may which up which may preven them from having a safe and successful mission.

JSC's Astronaut Quarantine Facility information sheet (direct link to 172K PDF), and a story about the building's construction.

Google Maps link.