- "Oh God... This is it. It's all over... This is how it ends. That's right... I have to end it. This nightmare will finally be over... Time to... wake up... My fingers scraped across the floor as I picked up the scalpel. I lifted it slowly, carefully, to my neck, as if someone were guiding my hand with theirs... And drew it across my throat."
- — Sigma, about to kill himself because of Radical-6
Radical-6 is a lethal airborne virus unleashed by Free the Soul that is introduced in Zero Escape: Virtue's Last Reward and an important subject of the story and the series. First appearing at the end of 2028, it caused a worldwide pandemic on Earth, almost eradicating humanity by suicide.
It is extremely contagious, requiring a quarantine for anybody afflicted by it. Radical-6 has two major symptoms.
The first symptom affects the brain, mainly a person's perception of time. An infected human perceives time at about √6 of normal rate, increasing mental processing rate. For every roughly 2.45 seconds in real time, an infected human would process this as 1 second. Thus, 2 minutes and 27 seconds (147 seconds) in real time is perceived as 1 minute (60 seconds) to a patient infected by Radical-6.
The second and truly fatal symptom is an overwhelmingly strong desire to suicide. It is strongly implied or believed that this is because of the virus' first symptom of the slowed processing rate. Because the brain's processing rate is slowed, they are taking in more information, and the brain can't keep up and thus, starts to fail. There is no premeditated method that they will kill themselves with. Most will choose simple methods such as shooting themselves, hanging themselves, etc. Usual rational thought is ignored, since depravity to kill themselves leads to crashing planes and mass unified suicide. The desire is so strong that even a small child can slightly overpower an adult trying to subdue them. The only possible way to stop this symptom is to knock them out with a strong sedative, such as Soporil ß.
Patients that are showing signs of Radical-6 will stand with their eyes blank and their mouths slightly hanging open as their processing speed is slowed down. To the patient, the world will appear as if it is on "fast-forward". Everything happening, including when someone speaks, appears to be sped up and can hardly be understood.
Radical-6 Nevada Outbreak
In 2028, Radical-6 first appeared after an accident occurred at the Mars Mission Test Site in Nevada and the participants become infected by this disease. It managed to kill 6 of the subjects. It is not known why the virus was present in this test site or how it had gotten in.
The disease escaped the test site in Nevada on December 31, 2028, and spread across the world, becoming a global pandemic. Bodies began to pile up in the millions and then billions as people mass suicided by any means, creating an extremely nauseating stench from all of the rotting corpses. The humans who were immune to the disease were locked in underground shelters.
By April 13, 2029, the death rate was so bad that the remaining humans plotted to eradicate the virus by blowing up 18 antimatter plants simultaneously. However, by doing so, the dust fallout blotted out the sun, killing millions of lives including plants and animals, and turned the Earth red.
In 2038, from several survivors who built up an immunity to the disease, Axelavir is developed as a cure for Radical-6 by harvesting antibodies from the immune survivors.
Virtue's Last Reward
In 2074, during the events of the AB Project on Rhizome 9, 8 participants of the game were infected with Radical-6. One was not infected because they are a GAULEM, thus immune to diseases. They were infected to help develop Phi and Sigma's timeline jumping abilities. It was also use to mask the fact that they were on the Moon, where gravity was 1/6th the normal strength. The disease's most deadly symptom were shown only through Quark and Alice. In most routes, Quark shows suicidal thoughts after going through the first set of Chromatic Doors and is quickly subdued with Soporil ß. Usually right after Quark is sedated, or before the second round of the AB game, Alice begins to show signs of the disease. In some routes, Alice does kill herself, while in others, she is knocked out as well. During Quark's ending, Sigma, Dio, and Clover discover a journal that details the Radical 6 infection and its effects on the human population. Phi discovers the journal is written on December 29, but has no year written on it.
Sigma also shows symptoms of Radical-6 in the Security room on Luna's route when Phi appears to speak faster than Sigma can comprehend.
Everyone possibly shows the suicide symptoms in Clover END, including Phi and ultimately Sigma. All participants except for Quark and possibly K (whose fate is unknown) die from suicide, using a scalpel.
In some endings, including Phi END, Quark and Alice are cured of their suicidal condition using Axelavir. After the Nonary Game is over, Akane gives all of the infected players Axelavir.