Cyber-pollen

Cyber-pollen were tiny particles that could convert compatible organic matter, living or dead, into new Cybermen. It could be suspended in, and control the movement of, water.

The exploding Cybermen sent by disseminated cyber-pollen into Earth's atmosphere. The cyber-pollen then fell to the ground as rain and infiltrated Earth's graveyards 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)