The strange and mysterious condition known as Paradise Sickness is believed to be a curse laid upon the Darcia family. It was placed upon the family when Lord Darcia I tried to open a fake, Noble's version of the door to Paradise. He's known to have "disappeared" doing so, and so a curse was placed on his family.
Lady Hamona, Lord Darcia III's beautiful lover, received the curse. The curse affected her because it is not limited to bloodline, but to the House of Darcia. The sickness seems to suck away the soul, leaving nothing but an empty space. Hamona is shown, with her hair covering most of her body, vacant, seemingly awake, but with the sickness present.
It is not clear if the Paradise Sickness is a common disease or of its status as a condition. Only that it comes from the world of Paradise. The only two people known to have been infected with Paradise Sickness were Darcia the III's lover, Hamona, and, possibly, Darcia III himself: He often talked to himself as cursed, even if his wolf's eye is a very different symptom from what struck his beloved.
Generally speaking, the whole Darcia family has been defined as cursed, due to Lord Darcia I's actions.