My favorite story about The Thing, other than the original story, posited that the blood brain barrier protected their minds from infection, or at least waylaid it long enough to allow the parasite to decide to keep them functioning normally. That every one of the infected was a sleeper agent, completely oblivious to their traumatic sublimation into a terror from the stars. That they were forced to watch as their body spontaneously revolted and maimed and defiled, only to be slowly consumed in the aftermath.
Viewing the body as a prison is one of my favorite horror tropes.