The Coalescence was a collective culture and species of microscopic organisms which explored the Alpha Quadrant in the mid 23rd century by infecting humanoid host organisms (referred to as "macroscopics"). Separate colonies could communicate telepathically across interstellar distances, allowing the Coalescence to remain as a single entity with rejected concepts of "Self". However, certain strains, if left in a host body long enough, could "go native" and develop a sense of individual existence.
A Coalescence colony infected Starfleet Lieutenant Theresa Errgang sometime prior to 2268, in order to investigate the strange healing powers of the Ilaiyenai of planet Sigma Niobe II. Following the conclusion of its mission, this colony transformed its viral components to mimic a strain of Andronesian encephalitis, and also removed any memories of its presence from Errgang's brain. (TOS short story: "As Others See Us")