The pod plant was a plant species whose flower contained thousands of microscopic spores. Originating from an unknown location, a number of these plants had drifted through deep space until they came to Omicron Ceti III, a planet constantly bombarded in Berthold rays, which the plants thrived on.
They were eventually discovered on the planet by botanist Leila Kalomi in 2264. The plant's spores infected Kalomi, Elias Sandoval, and the other members of the Omicron colony, after the spores were discharged from the plant's flower. The infected person then gained protection from berthold rays, as well as perfect physical health (even regrowing missing organs) and peace of mind. The colony's inhabitants who hosted these spores were then able to continue to live for many years in the environment that was otherwise deadly to Humans.
In 2267, the USS Enterprise arrived at Omicron Ceti III, when the colonists tried to infect the crew, which was exacerbated by an infected Doctor Leonard McCoy, who had numerous pod plants beamed up to the ship, where the spores quickly spread through the ventilation system.
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")