A prion is an infectious particle.
In 2376, Ethan Locken had developed a prion that he intended to infect several Alpha Quadrant species with. Locken's intended first target of the prions, designed to be introduced by a missile, were Romulan troops on Orias III. Locken's plan was stopped by Julian Bashir. (DS9 - Section 31 novel: Abyss)