The planet Omicron Ceti III is bombarded in berthold rays, but unfortunately the rays were not known to Federation science until 2267, after a few colonies had already been established on the planet and wiped out under unknown circumstances. The last colony, the Omicron colony of 2264, was protected by spores until those spores were destroyed by the actions of the in 2267. The colony had to be relocated because living animal matter could only survive on Omicron Ceti III for a week without protection. (TOS: "This Side of Paradise")
In 2343, Lenora was infected with the alien spores, the effects on her were classic, and relatively easy to remove. Yet the spores also affected her fetus, they bonded with it's developing DNA, and effectively became part of Vanessa Yates. At birth, the telltale signs of infection were apparent when she was born Hermaphrodite.