Prion

A prion was an infectious microscopic particle composed only of proteins. Naturally occurring prions were responsible for several diseases in plants and animals.

In 2373, Julian Bashir was studying prion replication in ganglionic cell clusters. Early that year Bashir and Jake Sisko attended a conference where he said "prion replication could be inhibited by quantum resonance effects." He would later explain his research to his mother, explaining that he was "working on two studies of prion replication in ganglionic cell clusters." A project which she felt he could do on Earth By the end of the year, he would explain to Jake and Nog, who wondered if he would rather be doing something else, he stated that that was the first chance he had to get any research done on his prion project in five months, and that "you couldn't drag me away from here." 

The same year, Hagath suggested the use of artificial prions, or likewise a mutagenic retrovirus, as a biogenic weapon, for killing up to 28 million of General Nassuc's species.