|“||It is a living thing. A thing to put an end to Human history||”|
–Chief Kakuzawa, in Chapter 66, revealing to Doctor Arakawa that his insane plans are very much for real
The Diclonius Virus, also referred to as The Vector Virus, is a mysterious disease which the Diclonius species spread to infect and displace humans through their vectors as a means of ensuring that their children are born as Diclonii.
In the series
The virus' nature is not well known or fully understood. It is both deliberately spread as part of the displacement effort and as a byproduct of any Diclonius attack on a non-horned human. As far as either the manga or anime relate, the virus only affects human males when spread through vectors, who then father Diclonius children. There is no data on how the virus specifically affects humans when airborne, such as how it affects men or whether it can now affect women as well. Possibly, the women became carriers, ensuring that any male near them, if somehow initially uninfected, still ultimately became so. Whatever the case, the result was that their children would be Diclonii.
The origin of the virus is the unnamed mother of Lucy, the first Diclonius Queen. This woman possessed a reproductive system capable of producing fertile Diclonii, as opposed to other humans only birthing Silpelits after being infected by vector exposure. According to an account by Chief Kakuzawa, after learning Lucy existed, her mother was found and kidnapped by the Diclonius Research Institute. Kakuzawa forced her to give birth to fertile Diclonii of his line, though it's never made concrete if it was through rape or forced insemination. Despite many attempts, she only birthed one child for him before she found a way to end her life. Efforts to create more children or extract the secrets of the virus from her remains were not successful.
Lucy likely infected the three bullying boys who were among her first victims, but since they're dead before the story begins, that speculation is moot. It is possible she infected Kouta with the virus, but this is unlikely since she is never shown using her vectors on him and the vectors likely can not pass genetic material through unbroken skin. In any event, by the time he became a father, there was already a vaccine created by Arakawa to prevent Diclonius births, and his child seemed not to be Diclonius. Another possibility is that someone at the Kamakura Festival survived her grief-stricken onslaught after coming into contact with her vectors, perhaps through nonfatal wounds. In any case, Lucy's first intended targets were not sought out until after her murder of Kouta's father and younger sister.
One scene in the manga flashbacks shows Lucy using her vectors to cause sudden heart failure in a woman, an act which draws a crowd. It is possible she used these groups to seed the men there with the virus. Also depicted, Lucy sends her vectors through people's bodies while walking down the street, killing some and infecting others; Lucy would later claim these actions as demands made of her by the DNA Voice within her. She undertook these attacks in secret for five years until Kurama, and the younger Kakuzawa realized there was a connection between the rise in Diclonius births and the increase in catastrophic circulatory failure in the Kamakura area, and their discovery ultimately led to her capture.
The degree to which Silpelits were able to spread the virus in this period is inconclusive, as those that weren't euthanized at birth or taken to the Diclonius Research Institute most often died during the violent eruption of their powers as they reached three years of age. One who was able to spread the virus was Number 3, who infected Kurama and Oomori, leading to them both to sire Diclonii. In particular, Kurama's infection brought about the birth of Mariko, the most powerful known Diclonius, and the only one known of to be two generations removed from Lucy's mother. This progression indicates that the virus mutates from generation to generation, but whether this is a stable or a radical mutation is harder to pin down. Mariko did not appear to age the same way as other Silpelits, looking much more like the five-year-old she was said to be, as opposed to looking twice her age like Nana. While Mariko possessed more vectors of greater strength than Lucy's, they were not so overwhelming that Lucy's cunning and ruthlessness could not bridge that gap, at least in the manga. In the anime, Mariko effectively dominated Lucy in an all-out battle. It seems like the virus created Diclonii of ever-increasing strength across the generations to ensure the replacement of non-horned humans so that a drop-off in population didn't also decrease their effectiveness.
Neither version details exactly what the virus is or how the vectors can transmit it. One possibility is that the vectors simply did carry the virus on them by some means well outside real world physics, which is possible due to the presence of soft science in the series already. Another possibility is that the vectors rewrite human DNA within enough cells in the targeted human male to create the virus within their bodies. A third option has the vectors establishing a connection between the bloodstreams of Diclonii and targets, leading to transmission by this method. Again, the series offers no guidance in this matter.
Viral transmission by individual Diclonii came to be a moot point once the master plan of Chief Kakuzawa came to fruition. Kakuzawa, believing that he and his family were distant relatives of the Diclonii, abused his position as head of the Diclonius Research Institute and ordered research into isolating and essentially weaponizing the virus, whatever its true nature. A large number of containers holding this virus placed aboard a supposed weather satellite that was designed to explode after launch went undetected. Immediately, nearly 20 million people (or 42 million in real life statistics based on the Japanese 2010 census) in the Kanto region of Japan were exposed. Within five months, the airborne virus would have spread worldwide. Ironically, neither Lucy nor Chief Kakuzawa lived to see the results of this plan.
According to an account by Kouta, enough Silpelits were born as a consequence of this infection to lower the human population of the Earth dramatically. Authorities implemented a birth ban instituted and adhered to until a vaccine developed by Arakawa was distributed, once more allowing for the previously infected to have non-Diclonius children. Arakawa based this vaccine at least in part on preliminary research done by the younger Kakuzawa, who wanted to use its threat as a 'leash' on Diclonii behavior. In the manga, Arakawa was more aware of the evil intentions of the DRI but wished to unleash the virus she created on humankind and become a "hero" by saving humanity from her creation. In the anime, Arakawa is more of a reluctant clutz than a mad scientist. Kouta's account places human extinction as being a very near thing as a result of this conflict, but no details exist on where and when the worst of it took place.The mistaken racial beliefs of the Kakuzawas, the harsh treatment of Lucy and all the other Diclonii, and the possibility of an evolutionary killer instinct made this virus as dangerous as any which directly and immediately affect the health of the victim.