Star Trek The Original Series Rewatch: "The Pirates of Orion"
Production episode 22020 Stardate: 6334.1 Captain’s log. There’s been an outbreak of choriocytosis on the Enterprise, but McCoy has it under control, so the ship can still fulfill its mission to attend the dedication ceremony for a new Academy of Sciences on Deneb V. However, Spock collapses on the bridge and is taken to sickbay. He’s contracted choriocytosis, and it’s fatal to beings with copper-based blood. The nearest supply of the only drug that can cure him is four days away, but Spock will only live for three. (Why the Enterprise didn’t stock up on the drug when the outbreak...
Source: tor.com