The Tarellians were a humanoid, spacefaring race native to the planet Tarella. The Tarellians had attained a level of biotechnology comparable to late 20th century Earth, when a civil war caused deadly biological weapons to be unleashed on the planet, wiping out the Tarellian civilization.
A few plague-carrying Tarellians managed to escape to other planets, where they infected the native population and died with them. The remaining Tarellian starships were hunted down and destroyed by fearful species. The race was believed to be extinct when the Alcyones destroyed the last known ship in 2356.
The last recorded plague ship arrived at Haven in 2364, seeking an isolated site to take advantage of the planet's supposed recuperative powers. They were joined by Wyatt Miller, who hoped to finally find a cure, and decided not to land at Haven. (TNG: "Haven")