Residents of Steveston, Oregon who had been implanted with immature cloned symbiotes were referred to as nightwalkers. Scientists Richard Flemming and Peter Stofer had worked for Immunitech Research, a wholly-owned subsidiary of Adrian Conrad's Zetatron Industries, based in Phoenix. They had cloned the symbiote that was implanted into Adrian Conrad, hoping to create a kind of super-drug.
One week after Adrian Conrad was taken into custody, Immunitech's operations were scaled back and moved from Phoenix to Steveston, a small town on the Oregon coast. In a lab accident there, Flemming and other townspeople became infected by the symbiotes. The cloned symbiotes had no naquadah in their blood, and were too immature to assert permanent control over the host. They became active only at night, leaving the unsuspecting hosts with no memory of their activities in the morning. Each night the nightwalkers gathered at the abandoned shipyard to build an alien spacecraft in order to escape from Earth.
After ten months, the spacecraft had been nearly completed, and for the last three of those months, NID agents had been monitoring the situation in an officially sanctioned operation to acquire the alien technology. When the nightwalkers became aware of the NID surveillance, they intended to implant NID headquarters with symbiotes, but their plan failed and the town was secured by NID agents. The cloned symbiotes had been engineered with a susceptibility to a particular sulfa-based antibiotic which was administered to the townspeople under the guise of an inoculation against meningitis and successfully eliminated the symbiotes. The alien ship was transferred to Area 51.
Episode Reference: Nightwalkers