Simon Verstraete shows how to create an infected system in Houdini using Pyro. 

This new tutorial from Simon Houdini (Simon Verstraete) shows a setup for growth and infections in Houdini that uses Pyro to spread the elements. The tutorial also covers particle simulations, collisions, and color.

A little while back, VFX Artist Kate Xagoraris showed how to use Pyro Source Spread SOP to create a similar animated growth effect.