From the introduction:
Two of you will be Brain Bug Brood Queens. From deep in the earth, the Brood Queens have sent forth their warriors to infiltrate the brains of humans to defend their age-old dominion over Earth.
Two of you will be playing Cyber Mind Mainframes, sent to Earth to include the planet in a universe-wide network of racial computer, calculating the Great Questions. The Mainframes upload Trojans to take over human minds and add them to their "wetnets" of biological terminals.
One of you will play the Human Host. The human host represents the myriad of human life, the humans that play hosts to the battling forces.