Star Trek: The Original Series, season 1, episode 29
Operation – Annihilate!
Summary
The Enterprise arrives at Deneva - the home of Captain Kirk's brother, Sam, and his family - and discovers that the entire planet has been infested with large, amoeba-like aliens that have attacked and killed much of the human population. One of these aliens attaches itself to Spock, who volunteers to become a subject in Dr. McCoy's medical tests. McCoy and Kirk find a cure in time to save Spock and the remainder of the Denevan population.
Summary from Wikipedia, CC BY-SA 4.0.
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