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Zero Day (ゼロデイ Zero Dei) is the name given to the day of the first Bugster outbreak 5 years ago.
Little is known about the details, but Zero Day's outbreak was caused by a glitch in the 10 games Genm Corp. had in development at the time that spawned the Bugsters in the first place. It is hinted that despite their use in combating the outbreak, the Prototype 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, and Kiriya Kujo's friend Jungo, 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, but left Seito University Hospital after losing his medical license when he failed to defeat Graphite, leading to Saki's death. Though Kuroto Dan was the real mastermind behind Zero Day, he framed his father, Masamune Dan, who was later 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.
- 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.