Zero Day is an event which saw the first outbreak of the Bugster Virus in Kamen Rider Ex-Aid.
History[]
Leadup to Zero Day[]
In the year 2000, the Bugster Virus first emerged from the Y2K bug and was discovered by Masamune Dan, CEO of the video game company Genm Corp., and his 14 year old son Kuroto, who was the lead programmer at the company. Taking an interest in the virus, Masamune would deliberately infect himself with it in order to develop the "ultimate antibody" within himself.
Kuroto would later receive fan mail from a young Emu Hojo, which included his ideas for a game in Genm Corp.'s Mighty series called Mighty Brothers XX. Furious that Emu was able to come up with game ideas that were comparable to his, Kuroto responded to his mail by spitefully sending him the Y2K Bugster Virus, causing Emu to become unknowingly infected by it.
First Zero Day[]
In 2011, once Emu had matured and the Bugster Virus had developed inside of him, Kuroto employed the Next Genome Institute headed by Michihiko Zaizen to extract the virus from him. Kuroto would then disseminate the virus to people through the Proto Gashats, triggering a massive outbreak of the Bugster virus.
While most outbreaks of the Bugster Virus were contained, it still claimed several victims including Saki Momose, the girlfriend of Dr. Hiiro Kagami, Jiro Maki, a radiologist at Seito University Hospital and colleague of Taiga Hanaya, and Genta Uesugi, the son of police detective Heiji Uesugi. Prior to Zero Day, Kuroto had also infected his mother Sakurako, who was already dying of an unknown illness, with the Bugster Virus for DoReMiFa Beat so he'd be able to hold on to a piece of her after she died in the form of the Bugster that emerged from her.
Aftermath[]
To avoid being caught, Kuroto framed his father as the one responsible for Zero Day. Secretly, Masamune had been aware of his son's plans all along and allowed himself to be framed in order to absolve him of responsibility for the future crimes his son would commit in regards to Kamen Rider Chronicle.
Five years after Zero Day, Kuroto would begin working with Parado, the Bugster Emu had held inside of him, to kickstart another infection in order trap humans within Kamen Rider Chronicle. The Graphite Bugster attempted to use the Proto Drago Knight Hunter Z Gashat to spread an infection comparable to Zero Day, but was stopped by the Bugsters.
Once development on Kamen Rider Chronicle was completed, Masamune arranged for the truth to come out about his son's plans and was released from prison, enabling him to retake control of Genm Corp. and put in motion his plans to use Kamen Rider Chronicle to dominate the world.