A pathogenic prionic organism (PPO) is an organism of terran design.
PPOs were used to infect hydralisks as part of Project Tamed. The PPOs 'tricked' their hosts into growing a new cerebral lobe. This allowed the hydralisks to supposedly be controlled by their overseer. In reality, the control was ineffective, and the hydralisks remained under the control of the Queen of Blades.