Genetically engineered viruses that destroy DNA of persons exposed to it. Metagenic viruses were designed to mutate rapidly, wipe out entire ecosystems within days, and then die out after a month. Those using such weapons could therefore slay entire planetary populations without destroying the planet's infrastructure. The major galactic powers, including the Federation, Ferengi, and Romulans agreed to treaties banning such weapons because of the danger they posed.
In 2369, Starfleet intelligence believed that the Cardassians were developing new metagenic weapons that would be delivered to their targets via theta-band subspace carrier waves, but this proved to be deliberate misinformation spread by the Cardassians.