Jack had plotted to lure the person behind Frazer Labs--the group that ordered the plague pathogens from National Biologicals--out into the open so he could at least identify them. In Chapter 32, he is sitting in his rented cargo van at a short distance from the mail box shop Frazer Labs uses as an address. In the surrounding dark, he finally sees a nondescript man enter, then leave the shop with Jack's faked Fed Ex package in hand. Jack follows him on foot.
The man disappears into the interior of an all but deserted commercial building that Jack studies from the sidewalk from where he sees a light appear on the fifth floor (ironically the same floor as Jack's office in another building in another part of New York). After Jack lingers from some hesitation about how to proceed, the same man hurries back out of the building in some agitation. Standing near the entrance door, Jack sticks his foot in the doorway to hold the door ajar and slips in after the man is out of view.
Jack's quest for information takes him to the roof and down the fires escape to the fire landing outside the man's fifth floor window (remembering that Jack is afraid of heights). With some nervousness, Jack climbs in through the open the window. What he sees is a home in combination with a fully equipped microbiology lab, complete with captive animals in a shed in a far corner of the warehouse like fifth floor loft. Situated in the lab is a freezer box--cooled to enormously low degrees by a nitrogen set-up--like the freezer box Beth investigated in the Manhattan General microbiology lab, which was an action (instigated by Jack) that got her killed.
As Jack is about to find his way out, the man comes back and by locking the key-only dead bolt, locks Jack in the loft.
The deadbolt had no knob. A key was required from both inside and out. Jack was locked in!
Previously, Jack had confirmed that this home-lab was indeed Frazer Labs, but now there was no place to run to and no place to hide except in the bathroom. When he is found, he is shocked to see the man is Richard, a lab technician from Manhattan General. We know Richard as one of the two people who caught Beth in the microbe freezer at General; Martin and Richard were the two who caught her.
Richard handcuffs Jack to a supporting column in the loft and calls his accomplice. When the accomplice angrily enters the loft some time later, it isn't Martin nor Zimmerman nor Kelly, all from Manhattan General Administration. With the gun in Richard's hand pointed at him, Jack anxiously recognizes the sound of the elevator and waits to see who will come through the door, then meets with surprise.
Jack took a breath and turned his head as he heard footsteps close in on him. When he caught sight of who it was, he gasped.
In the midst of all this, Jack's flu symptoms (a contagion he caught from Gloria Hernandez when he went to see her) are worsening while he diligently continues to take his anti-viral rimantadine.