The Vegan D virus was a life-threatening virus.
Samples of the Vegan D virus were being kept in the USS Shepard's bio-sterilization capsules when the Whale Probe disabled the ship in 2286. The Vegan D virus escaped containment and was believed to have infected the entire crew, forcing the captain to quarantine all but a minimal support crew. It was responsible for the deaths of fifteen crew members at the time of the Shepard's distress call to Starfleet Command. (Star Trek IV: The Voyage Home)