Real World article
(written from a Production point of view)

According to the script for Star Trek IV: The Voyage Home, a bio-sterilization capsule was a type of eletrical containment device used for pathogens.

During the Whale Probe crisis of 2286, the starship USS Shepard was transporting the Vegan D virus in these capsules when the ship ran afoul of the probe. The encounter resulted in these capsules failing, and fifteen crew members dying from the virus.