The Virogen was a deadly disease that was created in the 24th century.

Developed by the secretive Symmetrists movement, the Virogen was a single helix strand of RNA that had no cell membrane. When infecting an animal, the virus was harmeless but kept the host as an infected carrier. However, when infecting a chlorphyll producing plant, the Virogen attached itself to the chloroplasts where it used its energy production to fuel the growth of the virus. This led to the slow decay and loss of plant life which was part of the Symmetrists plot to devastate the Federation in order to strengthen their view about respecting the life of individual worlds.

Amongst Human's that were infected by the virus, they suffered symptoms which included fever and abdominal cramping during the first thirty hours after which the person returned to normal but remained as a carrier. Cultures of the Virogen were being cultivated at the Symmetrist stronghold on Tarsus IV along with an anti-Virogen but after the release of the plague the cure was ineffective as the Virogen had mutated during its spread. However, Trannin leaves were capable of serving as a cure for the Virogen and was used to end the threat posed by the plague. (TOS novel: Avenger)