The development and maintenance of the virus was supposedly for the security of the human race, so that the ultimately docile Bioroids could be effectively eliminated, should they uprise or begin to overpopulate either the island of Olympus or the rest of the world. In a sense, it was marketed as both a stop-gap and "reset button" should the experiment of genetically modified humans go wrong.
The real truth, however, was quite the opposite; the actual nature and formulation of the virus acted in reverse, and only affected human biology. The intent for this, was should humanity prove uncooperative with their Bioroid counterparts, or should the stirrings of yet another new global war begin, that species could be discarded and the Bioroids favoured instead for global repopulation. This, as it was later seen, was ultimate goal of the quietly-renegade upper house of the Olympus Legislature, known as the Seven Elders, which had wanted to see this plan realized as early as the 2110s, while the Earth fought the final battles of World War IV, and the metropolis (or "arc," as it would be) of Olympus neared completion.
In the 2004 Appleseed movie, it was said that Dr. Gilliam Knute was responsible for D-Tank's creation, and that she was murdered byorder of the Elders when she refused to capitulate to their plans and unleash the virus in this earlier time period. This was realized by manipulating the Bioroid resistant military arm of General Administration Agency, simply by steering them in the direction of Knute's secret research laboratories, located at sea. Resisting the destruction of a set of incubators, likely housing a prototype generation of infant Bioroids, she was shot dead. Edward Uranus appears to have been leading the insurgency, making him a pawn of the Elders on two occasions. ESWAT then proceed to disable all manual releases towards D-Tank. However, the Elders had one last trick up their sleeves: the ten mobile fortresses stationed around Olympus begin marching towards Tartarus, intent on destroying D-Tank. Though it use to be said that security around D-Tank was impenetrable, a close-range shot from one of the main gun turret could breach the system. ESWAT mobilized to stop the mobile fortresses and managed to stop them at the last minute.
Depicted as a green liquid, the D-Tank virus was an airborne pathogen that would spread widely and cause a 100% infection rate in its targeted victims. It affected the race it had been formulated against by irreparably damaging their reproductive systems, thereby guaranteeing their total elimination, as the generation present upon release of the virus would be the last. However, it wouldn't kill the infected individuals outright, abiding by the Elders' decree that, in their genocide of humanity, "not a single soul should be harmed." "Instead, we will slowly close the book on Humanity."
It's indicated that several thousand litres of the virus is enough to contaminate the globe.