Helena

Helena was a planet that was home to a Federation colony consisting of varied species in the mid 24th century. In 2370, it was turned over by the Federation to the Cardassian Union, and placed within the Demilitarized Zone between the two powers.

In 2371, a plague struck Helena, similar to one that had hit Archaria III and Bajor. The Cardassians threatened to sterilize the entire planet to prevent the disease from spreading. In an effort to alleiviate the illness, Maquis fighter intercepted a shuttle being piloted by Lieutenant Thomas Riker of the USS Gandhi carrying medical supplies from Outpost Sierra III. Riker was taken to Helena and, witnessing the situation there, decided to join the Maquis and use the medical supplies to help the colonists. (TNG novel: Double Helix: Quarantine; : A Weary Life)