Deadly biomechanical gene-disrupting weapons used for centuries in the war between the T'Lani and the Kelleruns, destroyed with Starfleet help in Armageddon Game.
The Harvesters are engineered weapons that attack a victim at the genetic level and kill within a short time. Both the T'Lani and the Kelleruns used them in a long and devastating war, and when peace came the two peoples agreed that every sample, and all knowledge of how to make them, must be eliminated.
In the Deep Space Nine episode Armageddon Game, Julian Bashir and Miles O'Brien spend a week helping scientists from both sides to neutralise the last stocks. As the work ends, soldiers kill the research team, since anyone who understands the weapons is considered a danger.
Bashir and O'Brien escape to the surface, but O'Brien has been splashed with a Harvester sample and grows steadily weaker. The two governments send a faked recording of their deaths to Sisko, and a small mistake in it noticed by Keiko leads to their rescue.