A deadly form of radiation that breaks down animal tissue after prolonged exposure, bathing the colony world Omicron Ceti III in the Original Series episode This Side of Paradise.
Berthold rays are a type of radiation discovered not long before the 2260s and still poorly understood at that time. Continued exposure causes living tissue to disintegrate, and no one is expected to survive it for more than a short period.
In This Side of Paradise the Enterprise travels to Omicron Ceti III, expecting to find that Elias Sandoval's agricultural colony has perished under the rays. Instead the colonists are alive and in perfect health.
The explanation lies in alien spores that live in the planet's plants. They shelter in a host's body and protect it from the radiation, while also inducing a blissful contentment that leaves the colonists without ambition and soon affects Spock and most of the crew. Kirk discovers that strong emotion drives the spores out.