The Ua Virus

The Ua virus is a biological weapon of mass-destruction that kills its victims within twelve hours.

428: Shibuya Scramble
The virus was created by an unknown country to be used as a weapon and it quickly became the most dangerous virus in the world until an anti-virus was eventually developed by Kenji Osawa. This, however, lead to a terrorist organization called "Snake" to try and steal the cure for themselves in order to make profit. Their mission ultimately failed, thanks to the actions of Minoru Minorikawa and a handful of others.

A second generation of the Ua virus was later developed by a joint venture between the CIA and Snake.

Borners
Borners are people who have been infected with the second generation Ua Virus, but have managed to survive. Upon survival, there is a chance that the victim's body will mutate and possibly lead to the development of terrifying abilities. An example of this is Hakko, who can inflict brain damage simply by speaking.

Regardless if the victim's body mutates or not, however, a cluster of their blood vessels will push towards the surface, causing a violet-blue mark to appear. In addition, victims must take special medicine to suppress the virus or they will eventually die.

Unblooms
Unblooms are people who were infected with the original Ua virus, but managed to survive. Like Borners, Unblooms possess violet-blue markings on their body. They do not gain any special abilities, however. Instead, they become extremely sensitive to UV rays, sunlight, and sudden noises. To combat this, Unblooms usually wear mascot heads to cover the entirety of their head and block out any foreign stimulation.

Trivia

 * 1) Ua means "flower of death" in Swahili.
 * 2) This is reinforced by the fact that victims develop violet blue marks on their bodies, resembling a flower.