Theses parasites fed on duranium alloys and were specially engineered to only target the linkages between segments of the Varro generational ship, before migrating to the USS Voyager in 2375. The growth of the parasites could be slowed with a polaron surge.
Derran Tal was one of those renegades; schematics of the parasites were found in her quarters. She and the other renegades hoped to gain freedom from the large generational ship by partially dismantling it and taking some of the smaller Varro pods with them to freely explore space on their own. (VOY: "The Disease")