The spores in the early drafts were a communal intelligence; when someone was possessed by them, that individual was granted telepathic abilities to link up with other possessed minds. The abilities of the spores to restore health were complete enough to enable them to return the dead to life. The antidotes for the spores were either the possession of a certain blood type or the introduction of alcohol into the affected person. Originally, Kirk leaped onto Spock and forced liquor down his throat to restore him to normal. This was presumably deemed unrealistic for various reasons including Kirk would not be strong enough to force alcohol on Spock and even if he did, Spock could just spit it out because the alcohol would have to enter the bloodstream to have an effect. It is established in various stories that, while Vulcans will occasionally drink alcohol, it doesn't affect (intoxicate) them in the same way it does a human. (On the other hand, in the novelisation of 'Star Trek IV: The Voyage Home', Spock states that the sugar sucrose, in the candies that Kirk bought to get change for the bus, has the same effect on Vulcans as ethanol does on humans.) In a surprise ending, the spores were revealed to be benevolent, conscious entities who never intended to act against anyone's will.
Spock hints that, contrary to the common misconception that Vulcans have only one name, he has more than one name like most humans, but when asked all he says about it is: "You couldn't pronounce it."
The empty shot of the bridge, before the turbolift opens to admit Kirk, was the best available piece of film for Relics (1992) to reuse as the holosimulation of the NCC-1701 bridge. The short snippet of film was "looped" several times and bluescreened in behind James Doohan. Using the stock footage in this way eliminated the need to completely rebuild the bridge - they only built a short section of the computer stations, the door alcove, and the command stations for the TNG-era actors to sit at.
In a blooper, Leonard Nimoy flubs his line about the plants acting as a repository for thousands of spores. Instead, he says the plants act as a "suppository." The crew cracks up, as does Nimoy, who caps the fun by putting a Tootsie Pop in his mouth.
This is the first episode in which Spock is shown to have superhuman strength.