The Omicron spore was the designation given to a spore encountered on the planet Omicron Ceti III. The spores' actual origin is unknown, except that they drifted through deep space until they came to Omicron Ceti, a planet constantly bombarded in berthold rays, which the spores thrive on.
The spores are held in repositories called pod plants, which discharge the spores into the face of anyone who comes close. The spores infect a person, protecting them from berthold rays, and also giving them perfect physical health and peace of mind. In 2264, the spores infected Leila Kalomi, Elias Sandoval, and the other members of the Omicron colony, saving them from the destruction that had been visited upon all other colonies on the planet.
In 2267, the USS Enterprise arrived at Omicron Ceti III, and the colonists there soon spread the spores to the crew of the starship, causing almost the entire crew to desert and beam down to the colony. Only Captain James T. Kirk remained unaffected, and he was able to deduce that his apparent immunity was due to the intense emotions he was feeling. These emotions destroyed the spores, and Kirk was able to incite anger in his science officer, Commander Spock, to free him from the spores as well.
Together, Kirk and Spock built a subsonic transmitter that they used to induce anger into the entire colony at once over the Enterprise crew's communicators, wiping out the spores. (TOS: "This Side of Paradise")