Silicon-based parasites were created by renegade Varro scientists.
The parasites fed on duranium alloys and were specially engineered to only target the linkages between segments of the Varro generational ship. The growth of the parasites could be slowed with a polaron surge.
The 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.
Derran Tal was one of those renegades; schematics of the parasites were found in her quarters. (VOY: "The Disease")
The parasites looked remarkably similar to carbon-based bacteriophage.