Medusa

One pivotal role in the series is that covered by the Medusa virus, a mortal disease so named after the Medusa from Greek Mythology, the Gorgon whose eyesight could turn anyone and anything into stone at a mere glance. The virus itself is extremely virulent, infecting its victim's cells and causing seizures while drying up the body, turning the infected into a solid, stone-like corpse.

Process

 * Stage 1: Incubation Period: 30 to 60 days from infection to onset.
 * Stage 2: 12 hours until petrification.
 * Stage 3: 100% fatality rate.

Background
While perceived as a terrible malady by the world, in reality the Medusa virus is not a virus at all, being a shapeless presence brought to Earth from outer space. It landed in Siberia during a meteor shower, by chance near a young boy and his pet deer, enough to instantly infect both him and his animal. Unknowingly bringing the concentrated thing to his home, he infected his whole family and his sister Alice. She unknowingly uncovered the true nature of Medusa when her imaginary friend, a cat-boy hybrid, came to life by erupting from her back. Terrified by the death of her family and the fact that the newborn creature devoured her brother's deer, she trapped it in her house and set it on fire, thus spreading Medusa all over the world through the fire's smoke.

It was then that the people affiliated with Venus Gate, a religious sect, showed themselves and approached Alice, believing her ability to turn imagination into reality to be a gift from the heavens. Experimenting on her and Medusa, during that time they assumed a hacker named Zeus as their security specialist, though in doing so they doomed themselves when he, pursuing his crazy dreams, developed an artificial way to force dreams into suitable hosts and, thus, fabricate mind-created realities at will to accomplish his plan to force the world into a primal survival game to amuse himself.

Turning Imagination to Reality
Three conditions must be met for Medusa to turn the figments of your imagination into reality:
 * 1) The infected person must have a strong visual imagination. The shapeless entity needs to find an image within the host's mind that it can adopt. After that, Medusa will awaken and create its material body using that image. Medusa will also take over a part of the host's body and then it will start to separate itself. However, if the image is not strong enough, the separation will fail and the creature will disintegrate. This will also affect the host, which will petrify.
 * 2) The second condition concerns the concentration of Medusa. The probability of getting infected by Medusa is proportional to its concentration. A low concentration makes it impossible for the first condition to be met, as a weakend Medusa cannot infect a human with a strong imagination.
 * 3) The energy needed for Medusa cells to replicate can be obtained easily. As long as the mutated Medusa cells have an available energy source, they will keep on multiplying. So, theoretically, only one cell is the starting point for the creation of a huge body who will never age or die.