Prion

A prion is an infectious particle.

Julian Bashir was studying prions in 2373. 

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.