Riders

Riders: Riders are sentient viral colonies. They are invisible to the naked eye, and normally exist only as an infection within a living being. They are completely undetectable without advanced equipment. Individuals who have spent a great deal of time opposing the Riders say that there's an indefinable "ridden look" to the Riders' hosts that can be recognized by persons experienced with them. Absolutely nothing is known about the Rider homeworld. When the Riders infect a creature, they share its environmental preferences and limitations. The Oceanic Alliance in the Orion Arm is under attack by a large Rider invasion.