Metagenic devices are powerful biological weapons outlawed by most major governments in the 24th century. A metagenic device is a genetically engineered, airborne virus that destroys all DNA it encounters, and is capable of destroying entire planetary ecosystems within days.
A Threat force armed with such a device could introduce it into a target planet's atmosphere to rapidly eliminate all life - and resistance - while leaving all infrastructure intact. Within a month, the metagenic virus itself becomes inert, leaving the planet and its facilities free for conquest.
In 2369, Starfleet Intelligence falsely suspects the Cardassians of developing a metagenic device in a secret base under Celtris III. (TNG: "Chain of Command, Part II")