Nanospores were insidious Grigari weapons that were deployed against the Federation during the War of the Prophets in the alternate future that was created when the Red Orbs of Jalbador formed the red wormhole.
Nanospores are nanites that infiltrate biological cells where they destroy the body from the inside out and use whatever materials they have access to within the living organisms body to create more of themselves. Nanospores could not be blocked by biofilters and resulted in many worlds being uninhabitable. Starfleet was forced to impose a quarantine on the worlds so affected, to prevent the spread of the nanospore plague. (DS9 - Millennium novel: The War of the Prophets)
