First appearing in December 2028, it caused a worldwide pandemic and apocalypse on Earth in the early months of 2029, almost eradicating humanity, as it led to events that caused the deaths of 6 billion people.
In Zero Time Dilemma, Radical-6, in its liquid form, appears as reddish or pinkish liquid. There are no known characteristics of Radical-6 in its gaseous form.
Radical-6 is extremely contagious, requiring a quarantine for anybody afflicted by it. Its mortality rate is 75%. Radical-6 has two major symptoms (which can appear as early as 10 minutes or take 2 hours):
- 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 kill oneself. 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.
A Treatment Pod can slow down and ease the symptoms of Radical-6, but it cannot fully remove the virus, as it does not contain an antidote. There is only one known cure that can completely rid someone of Radical-6: Axelavir. However, even after being treated, some of the symptoms (most notably reduced brain processing) will linger for a while.
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. Phi displays the symptoms in Clover END.
Radical-6 Nevada outbreak
On December 31, 2028, Radical-6 was believed to have first appeared after an "accident" occurred at the Mars Mission Test Site (Dcom) in Nevada and an unknown number of people working there become infected by this disease. It is not known why the virus was present in this test site or how it had gotten in and out, but Free the Soul was responsible for its creation.
In Zero Time Dilemma, it is revealed that Radical-6 was released at an underground shelter, not Dcom. Radical-6 being an "accidental release" was a coverup by Free the Soul to hide the real way the virus escaped (deliberately by Mira, an accomplice of Zero). It is revealed that Zero (Delta) released Radical-6 for a 75% chance of killing a religious fanatic who plans to kill all of humanity in the future, and he sees it as the lesser of two evils.
The disease escaped from the test site on January 1, 2029, after a young woman infected with the virus, Phi, was hospitalized. The virus spread across the world like a contagious wildfire, becoming a global pandemic, ensuring complete global saturation.
Radical-6 killed humanity violently, relentlessly and indiscriminately, no matter how old or young they were, no matter how what they looked like or how physically attractive they were, no matter how "good" or "bad" their actions in life were, no matter their ethnicity, no matter what language they spoke, no matter how much money and wealth they had, and no matter where they lived on planet Earth. In this sense, Radical-6 was an equalizer: death.
Bodies began to pile up in the millions, and then billions, as people took their own lives by any means in a mass suicide, creating an extremely nauseating stench from all of the rotting corpses. Nearly every single town and city on the planet had thousands - hundreds of thousands - of bloody dead, rotting corpses in giant piles and mountains. Radical-6 killed about 2 billion people.
- "Pretty soon the bodies started to pile up. Some people would take a dive off a building or shoot themselves in the head. You know, the classics. But for some it was poison, or fire, or asphyxiation, or the noose... Plenty of people jumped in front of trains or cars. One pilot on a passenger airline drove his plane into the ground."
- — Tenmyouji on the Radical-6 outbreak
Most forms of transportation were rendered useless as roads, streets, and highways became blocked by crashed vehicles, meaning food couldn't be distributed, so many people not infected starved. It became a literal hell on Earth, with humans killing each other for food and water just so that they wouldn't starve to death. The humans who were immune to the disease were locked in underground shelters by the government.
- "Most forms of transportation ground to a halt pretty fast. That meant food couldn't get distributed, and thousands of people who weren't even infected died from starvation. Everywhere you went there were just... piles of corpses, covered in flies and rats and crows. I don't know what Hell's like, but I imagine it's a lot like that."
- — Tenmyouji on the Radical-6 outbreak
Antimatter Plant Explosions
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 in a set of Antimatter Plant Explosions. It was successful.
However, by doing so, the dust fallout blotted out the sun, killing millions of lives including plants and animals, and turned the Earth red. This is actually what killed the majority of humans and animals, not directly from Radical-6 itself.
- "You ever heard of a nuclear winter? Well, that's basically what we got. The explosion threw a ton of crap up into the stratosphere, where it joined up with smoke and ash from all the fires. It blotted out the sun for seven years. Plants withered, and animals died by the millions. The few humans who were left banded together to survive. They were rough times."
- — Tenmyouji on the Antimatter Plant Explosions
As a result of the antimatter plant explosions, the spread of Radical-6 was successfully contained and the pandemic came to an end. After the Radical-6 pandemic, the ensuing chaos and the antimatter plant explosions, approximately 6 billion were dead.
Humanity then decided to expand to the Moon by building rhizomes - at least 54 of them. These rhizomes were deep underground and equipped with pressure exchange chambers in order to prevent Radical-6 from leaking in.
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 January 2074, during the events of the AB Project on Rhizome 9, 8 participants of the Nonary Game: Ambidex Edition were infected with Radical-6. Luna was not infected because she is a GAULEM, thus immune to diseases -- instead, her speech and body movement were slowed down in real time to give the illusion.
It is also unknown how Zero Sr./Akane Kurashiki/Crash Keys were able to keep a sample of the virus, despite it being destroyed in 2029, although Akane may have received it from Delta during her meeting with him.
During Quark END, Sigma, Dio, and Clover discover a Latin Journal that details the Radical-6 infection and its effects on the human population. Phi discovers that the journal was written on December 29, but has no year written on it.
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.
- "We're all going to die... We're all dead already... All of humanity is going to DIE! The virus will spread! Adults! Children! Everyone! EVERYONE! There won't be anyone left! I... I'd rather die here!"
- — Alice, on Radical-6
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, however he feels no suicidal urges and seemingly recovers after resting for a bit.
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 the infirmary. If K was infected, he would have found a different way to kill himself because the scalpel wouldn't be able to penetrate his armor.
- "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 Klim, slitting his throat because of Radical-6 in Clover END
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, curing them all of Radical-6.
Zero Time Dilemma
During the Q-Team's investigation of the Biolab, they discover files on a lethal virus, Fanatic Bio R. The only known antidote for it is Radical-6, which counteracts it, and at the same time is neutralized by Fanatic Bio R.
Sean, Mira, and Eric are informed after solving the room's puzzles that they could have been infected with Fanatic Bio R during their search and that a vial of Radical-6 was readily available for them to use should they be infected. Zero II claims that he knew who would inject themselves with Radical-6 beforehand and prepared their specific needle earlier to not contain the Fanatic Bio R. Q-Team decides whether or not to inject themselves with it.
In one potential timeline, they do inject themselves, but nothing apparently occurs. However, should they choose not to inject themselves with it, Mira smuggles the syringe with her out of the lab. Later on, it is heavily implied that Fanatic Bio R is fake and that it is a ruse to get Radical-6 released.
Later, D-Team decides to blow a hole open in their Decontamination Room using a bomb from the Manufacturing Room, as it leads to the Central Elevator Hall. Should Sigma send Diana and Phi away to take care of it, the bomb explodes, claiming both of Sigma's arms as well as his right eye. Their plan to use the bomb is successful and they make it into the elevator hall, where they find Q-Team dead, except for Mira who is nearly so.
As Phi attempts to check on her, Mira takes out her Radical-6 syringe and injects it into Phi before succumbing to her wounds. Now infected with Radical-6, Phi opts to kill herself and have her body incinerated to prevent the virus from escaping the facility. However, Diana rejects this plan, uses the stun rod on her, and drags both Phi and Sigma to the surface.
Outside of the facility, Diana contacts the Dcom supervisors and informs them that the 6 other members in the shelter have been killed. By bringing Phi to the surface, they have inadvertently exposed the world to Radical-6, thus setting the events of Virtue's Last Reward into motion.
- It is possible that the reason the virus was designed to make people kill themselves was because of the police ruling on Left's death, which was falsely accepted as a suicide.
- It can also be theorized that Delta may felt suicidal feelings after Left died and after being exposed to the negative side of humanity, and that Radical-6 was developed as a suicide virus so that all of humanity could experience similar feelings of pain and despair.
- Quark tries to kill himself and says that his soul has to escape. This could be a reference to Free the Soul's philosophy of "freeing the soul from the body".
- It is unknown if Radical-6 affects animals other than humans.
- According to a file in Zero Time Dilemma, Radical-6 did not kill 6 billion, it was the nuclear reactors that did.
- In Virtue's Last Reward, after listening to the recording on the memory card, Tenmyouji says two billion died directly from the disease, while the remaining four billion deaths were caused by collapsed infrastructure and other indirect effects of the enormous death toll, and presumably the subsequent antimatter plant explosions.
- The "radical six" may be an allusion to the six major espers at the end of Zero Time Dilemma: Junpei, Akane, Sigma, Phi, Carlos and Diana.
- A bit of a plot hole, the series never explains how Sigma, Phi, Diana, Akane and Junpei survived the Radical-6 outbreak, especially considering how 3/4 of humanity is affected by it.
- It is possible they received an immunization to it by Free the Soul somehow around the time of their abductions from Dcom, which could mean they carry a dead form of the virus which does not harm them, but can still kill others who are nearby. However, this would contradict some of them being affected by it in VLR, which may mean Akane used an altered strain of the virus compared to the one in the Decision Game.