Cyber-pollen were cyber-particles that could convert compatible organic matter, living or dead, into new Cybermen. Able to infect computer systems and be transferred with a touch, it was a kind of living metal (AUDIO: Return to Telos) that had the appearance of water. Each particle contained the "blueprints" for building a Cyberman.
Groups of Cybermen sent by Missy exploded and disseminated cyber-pollen into Earth's atmosphere, falling to the ground as rain and infiltrated Earth's graveyards and morgues to convert the dead into a Cyberman army. Missy planned to use a second batch of rain to kill the living population of Earth and convert them as well. (TV: Death in Heaven)