Day 3: 3:00 P.M.-4:00 P.M. - S3-E3
Plot hole: In the scene when Jack and Nicole isolate the Singers, Jack is informed that they've been tested on site, and the results were negative for the virus, less than 15 minutes after being isolated. However, later in the day the results take several hours to be made available, and have to be processed elsewhere, in a lab. (00:17:45)
Chosen answer: Nobody knew, not even the writers, until at least halfway through the season when they were writing the end. They wanted to keep all options open.
Nick N.