Quoting Doctor McCoy's intercom conversation with Spock, mid-episode:
The disease is not transmitted by the men, Mister Spock. The cause is the area of space we're in. It's affecting the whole crew. The molecular structure of the brain tissues in the central nervous system are distorting. And the madness that affected the Defiant's crew will soon happen to the Enterprise. Now you've got to get this ship out of here.
I'm a little confused here...
Is McCoy offering Spock an official diagnosis and prognosis for the crews of the Enterprise and Defiant? Is he saying that the Defiant's crew died of brain tissue disease caused by the unusual spacial distortion taking place in this particular region? If so, does this mean that if the Enterprise hangs around long enough, everyone will die regardless due to the deleterious affects this distortion has on brain tissue?
The Theragen-derived antidote
McCoy unveils later would seem to indicate that a chemical solution can be used to keep this brain tissue malady in check.
It seems a little contradictory. Did the Defiant's crew all go crazy on each other, and that was the cause of death? It looked like someone let the air out of the ship's interior, and Kirk's boarding party needed space suits with gravity boots to explore the derelict Defiant, suggesting there was no longer any life support available. Or did something else happen?