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Little is known about the details. Zero Day's outbreak was claimed to have been caused by a glitch in the 10 games Genm Corp. had in development at the time that the Bugsters' data had come from, but is later implied to have been intentional to spur an increase in the Bugster population. It is hinted that despite their use in combating the outbreak, the Proto Gashats were involved with it. The event was never made public, but it is implied many people were infected and died from the outbreak, including Hiiro Kagami's girlfriend Saki who was the host of Graphite, one of the Bugsters born from the outbreak; Genta Uesugi, the son of a police detective aware of the incident who pursued the case through to the present; and Kiriya Kujo's friend Jungo Aihara, who seems to have killed himself or otherwise died before the infection could fully take hold. Taiga Hanaya was a Kamen Rider during Zero Day, Proto Snipe, but left CR after losing his medical license when he failed to defeat Graphite, seemingly when he was temporarily removed from being a Rider and was directed not to operate, leading to Saki's death. Though Kuroto Dan was the real mastermind behind Zero Day, he framed his father, Masamune Dan, who was imprisoned.
5 years later, Graphite uses the Proto Drago Knight Hunter Z to spread a full-scale infections, creating a second Zero Day, even after his death at the hands of four Hunter Gamer forms of doctor Kamen Riders Ex-Aid, Brave, Snipe, and Lazer.
While interrogating Masamune about Zero Day, Detective Heiji Uesugi contracted the Bugster virus, leading Emu Hojo and Asuna Karino to Masamune, who tells them the truth about Kuroto being the cause of Zero Day after discovering the Bugster virus following the Y2K incident.
It seems as if another complete Bugster, Lovelica, was born from Zero Day but remained out of the spotlight until he was needed to help prepare the final addition to Kamen Rider Chronicle. His host is unknown.
It is later revealed by Kuroto that the data of all people erased by the Bugster virus, including the victims of Zero Day, is stored in the Proto Gashats.
After the defeat of Masamune, Emu holds a press conference where he lists all victims of the Bugster virus and vows to find a way to revive them from their data. The victims of Zero Day and related incidents are: Haruo Takaoka (previously unnamed, conditions and Bugster unknown), Genta Uesugi (Zero Day victim, unknown Bugster), Jiro Maki (overuse of Proto Bang Bang Shooting before Zero Day), Saki Momose (Zero Day victim to Graphite), Kiriya Kujo (killed by Kuroto for investigating too deep, later revival not revealed to the public), Sakurako Dan (victim to Poppy Pipopapo before Zero Day), Masamune Dan (former suspect, killed self to avoid punishment for Kamen Rider Chronicle incident), and Kuroto Dan (killed by Parado, later revival not revealed to the public).
- Zero Day's name is derived from "zero-day vulnerability", a term used in computing to refer to an undisclosed computer software vulnerability exploited by hackers, affecting computer programs, networks, and so on. It is usually not publicly known before becoming active, resulting in little to no time to resolve the issue and mitigate its actions, thus the term "zero-day". The fact that the Zero Day event was never made public, and that it was caused out of a sudden by a glitch in the games developed by Genm Corp. which led to the spawning of the Bugsters, further alludes to the "zero-day" term used in computing.
- The name also connects to the usage of the medical term "Patient Zero", a label for the first person infected in a disease outbreak.
- Despite this event claiming to be where the Bugsters first appeared, this is technically an incorrect statement as Asuna Karino is a Bugster herself and appeared sometime prior to the events of Zero Day.
- The virus itself seems to also have existed sometime before Zero Day's occurrence, as Emu Hojo was already implanted with the virus during a surgery one year before Zero Day.
- Episode 21 shows that the virus was created in 2000, partly due to the Y2K bug, and its true nature was hidden by Kuroto to avoid suspicion.