A prion is an infectious microscopic particle composed only of proteins. Naturally occurring prions are responsible for several diseases in plants and animals. Artificial prions designed as biogenic weapons are capable of causing millions of casualties when employed on a population. (DS9: "Business as Usual")
In 2373, Julian Bashir was studying prion replication in ganglionic cell clusters. He attended a conference where he said prion replication could be "inhibited by quantum resonance effects". He would later be forced to put his project on hold until the end of the year. (DS9: "...Nor the Battle to the Strong", "Doctor Bashir, I Presume?", "In the Cards")
Several weeks later, Hagath suggested the use of prions in killing several members of General Nassuc's race. (DS9: "Business as Usual")