The Ultra Bug was a reduced, non-deadly strand of the Dominion virus, created by the Styx. The Styx released it to show England their power, before they released the real Dominion virus to purge and reclaim the surface. Their efforts to launch the Ultra Bug were successful, although the virus eventually cleared out.
In Deeper, the Old Styx and Rebecca reveal their plot to reclaim the surface by killing Topsoilers with Dominion, their virus. The two release doves over London, each one carrying a bead which holds a strand of Ultra Bug. The sun melts the bead, causing the virus to infect England.
This strand creates havoc throughout England, and even spreads to other countries like the United States. Dubbed the "Ultra Bug," by Topsoilers, the strand reaches Humphrey House, where Mrs. Burrows is staying. It affects her and most of the House's residents. Despite what one man tells her, everything gets worse. Mrs. L dies, and to keep Topsoil from developing a vaccine to Dominion, the Styx burn down the lab researching the Ultra Bug. The media covers it up by saying an experiment, not arson, burnt it down. Making them restart working on the cure.
The virus strand presumably clears, as Mrs. Burrows gets better and so does the rest of London. However, the phial containing the Ultra Bug is with the Rebeccas, down in the Pore. In Freefall, they give Will the phial, keeping up the pretense that it is the real Dominion. When Will and Dr. Burrows reach Topsoil, they give Drake's friend Leatherman the phials for testing. Leatherman later reports back to the pair, saying the phials were Ultra Bug and not Dominion.
Later, the Ultra Bug phials are used in Drake's secret operation on Highfield Common to draw the Old Styx out. Mrs. Burrows is prepared to give the phials to the Styx in return for leaving her family alone. However, the mission fails, as Mrs. Burrows is captured and the Ultra Bug phials retrieved. Styx testing later confirms she did not have the true Dominion phials with her.
The Ultra Bug causes victims to have symptoms of coughing and swollen eyes, which make reading and looking at objects very hard. It is worse to children and elderly people; a woman named Mrs. L dies in Humphrey House after being infected.