Originating outside the Milky Way galaxy, these irregularly shaped gelatinous life-forms were structurally analogous to oversized brain cells, and were linked together to form a single collective intelligence.
Initially, the parasites hid in darkened recesses, but were capable of flight, and thereby mobile enough to attack a passer-by. Latching themselves onto a humanoid life-form, the parasites would then infiltrate the victim's nervous system assuming control of both the autonomic and higher functions of the inflicted humanoid. This process brought severe pain on the victim.
So virulent were these parasites that they could easily infest an entire planet's population, forcing the planet's unfortunate humanoid victims to construct crafts in which they might infest the next planet.
From their path beginning outside the Milky Way, the parasites first attacked the ancient Beta Portolan system followed by infestations at Levinius V, Theta Cygni XII, Ingraham B, and in 2267, they attacked the planet Deneva. Residing on Deneva were George Samuel and Aurelan Kirk, brother and sister-in-law to Captain James Kirk. Both of Kirk's relatives became victims of the parasite and died.
U.S.S. Enterprise NCC-1701 Science Officer Mr. Spock also became an unwitting victim of the parasites. Later, however, he was freed by an experiment involving a controlled exposure to intense light which successfully destroyed the parasites, and provided the means to destroy the remaining parasites on Deneva.
Two-hundred ten satellites were placed into orbit around Deneva, each bombarding the surface of the planet with enough powerful ultraviolet radiation to eradicate the entire infestation.