The Border Disease (極限病, kyokugen'in?, lit. "Border Disease") is a severe condition that only happens to humans that have been exposed to a significant emotional shock, or witnessed an extremely traumatic event. It has only been observed on the inhabitants of Sol Cluster.
This disease basically manifests as a loss of control of the person suffering it, along a dramatic increase in physical strength and resiliency. A few people can manage to overcome the symptoms and live with the disease, while benefiting from their enhanced capabilities. However, most of the patients end losing completely their sanity and humanity, and turn simply into empty shells who only live to kill and hunt other humans. These empty shells are known as Ultimate Border Humans, and are treated exactly the same as if they were monsters. Treating or curing these people is impossible at this stage, so the only solution for dealing with them is killing them.
The Border Disease has its origins on the Human Evolution Project that the Neo Atlas Faction of the Archia Think Tank developed to attempt to acclimate to the dying planet of Ar Ciel. This consisted in fusing the humans with Antibodies, using the Reyvateils as adhesive between both life forms. To do this, they first modified the Reyvateil genes, to make them active regardless of who was carrying them and then, implanted them into test subjects, which were Pureblooded Beta-type Reyvateils whose Telomere counts were set to only 20 years. Then, said subjects passed them down to their descendants, eventually spreading to all of the inhabitants of Sol Cluster.
These genes were supposedly for allowing the people to awaken as Reyvateils, and allow them to gain the ability to sing Song Magic. However, this was an utter lie, since these people, better known as Medleys, and technically known as Active Fusion humans, connect to the SH Servers, but this assigns to them Soulspace Addresses that the SH Server can't verify due to not existing. This normally causes no problems, but if the person undergoes an emotional arousal that would be enough for a Reyvateil to craft a Song, the SH Server tries to reach the Tower of Life inside of that person's Cosmosphere. Since it can't reach it due to the Soulspace having an invalid address, the Cosmosphere is deleted and replaced with a large stream of garbage data, which ends turning the people into Ultimate Border Humans.
Theoretically, it's possible to save a person that is still on the initial phases of the disease, but it has to be done as fast as possible. The only method to prevent a complete mental breakdown is through the redirecting of the defective Soulspace Address into one that directly connects to the SH Server, using the Cosmosphere of a Reyvateil as the intermediary.
Aoto also suffered this disease during the events in Ar tonelico Qoga: Knell of Ar Ciel, and he would have turned into an Ultimate Border Human if Saki had not offered herself to share her Soulspace with him, which ultimately ended saving his life. Hikari Gojo didn't approve such a procedure since the researchers of the Archia Think Tank were the ones performing it, but he also said that they didn't have any other choice.
As a final note, the cases in which the people got inflicted with the disease and managed to adapt to the symptoms comes from the fact that they were very lucky: the SH address they were assigned actually fall into the ranges that the SH Server does admit, which allows them pretty much to have the same abilities as a Reyvateil without them realizing it. However, this also entrails a handful of risks, as their Soulspace could get deleted at any time by the Garbage Recollection process the SH Server uses to clean up unnecessary data or whenever a new Reyvateil is registered into the Hymn Code Server (HNC), which would have the same end result as a normal case of Border Disease: the complete destruction of the Soulspace and the subsequent transformation of the victim into a empty shell who only lives to kill and destroy.