Cyber-nanotechnology was sentient, (AUDIO: Hour of the 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 containing 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)