The Houlats say that the plague came as a punishment when their ancestors made a pact with The Dark One, a monstrous deity, that is said, lives in the most obscure and hidden parts of the metro, praying on any man or woman that ventures too far from their station. His wills are carried on by His agents, the witches, so they must be erradicated, at any cost, as to prevent the further spreading of the disease through the metro.

The illness is caused by a bacteria, Necrotizing leprae. The diseased are usually affected at first by rashes, skin sores, and lumps and bumps in the skin, that doesn't seem to heal and are painful. As the disease advances, the synthoms worsen, and the areas affected tend to grow. The skin begins to break open and ooze fluid, and areas like the eyes, nose, ears, and mouth begin to be affected by the bacteria, which can eventually lead to blindness, hearing loss, and smell loss. Eventually, the victim will also begin to show fevers, sweating, chills, nausea, dizziness, weakness, and when the condition is too advanced, shock.

As the condition begans to worsen, which can take from months to several years, the bacteria will begin attacking the muscles, causing nerve damage, and eventually, the inner organs, such as the kidneys, liver, bladder, intestines, and stomach, which causes problems related to their respective functions.

Death normally comes as either a massive organ failure, or the failure of the heart and the lungs.

The disease, however, is curable. But so far, nobody has researched for a proper cure. The Underground Queen gives the Tunnel Chiefs a medicine that subsides the effects of the disease, but doesn't cures it. If the Queen has a medicine that cures the disease, it's currently unknown, as her origin or identity.