Malaise

The Malaise is described as a virus that has been infecting the island Dead Cells takes place on.

Origin
While the Malaise has no clear origin, The Alchemist, while he was searching for a cure, had been theorizing about where it came from. He states that some people think the Malaise originated from the Slumbering Sanctuary, and that the sap within there seeped into the Sewers, causing the infection from there.

Infection
The symptoms of the Malaise include vomiting blood, itchiness, headaches, blurry vision, bellyaches, and feeling down. Left untreated, the effects of the Malaise lead to certain death. Due to the fact that the infected were locked up in prisons and quarantined, the Malaise is probably highly infectious. The more extreme effects of the Malaise are much more unsettling. Mutations start to occur, and sometimes the bodies turn alive, moving on their own. The end results of the final stages of infection is the reanimation of the hostile, mutated undead.

Conjunctivius was likely the result of this mutation, and Castaing was a victim of the virus as well.

The Beheaded is suspecital to the effects of the Malaise, although it appears only the possessed body has this vulnerability. The "real" Beheaded is safe from the effects of the infection.

Treatment
Later into his research, The Alchemist managed to find a way to reduce symptoms from infection, as well as a method for slowing down mutation. It is implied that this way of slowing down mutations was created using the sap from the Slumbering Sanctuary, which he suspected could provide the cure.

In addition, by manipulating time, it appears that The Time Keeper has successfully kept the Melasia at bay, although this appears to be exhausting, and something that The Alchemist thinks she will be unable to keep up.

Effects on the Island
There is little sign of what the Island was before the Malasie made its debut, although it must have been at least somewhat prosperous. However, any chance at a good future on the Island has been ruined by the spread of the Malasie, and the actions of The King. The citizens of the island faced civil rest, and focused their rage on The King due to his reactions to the appearance of the Malaise. The King demanded that the infected were to be imprisoned and hanged, even just upon the suspicion of infection. Eventually, he demanded that nobody was to be let out of the prisons, even if they served their time. WWhat little of the villagers who remained on the Island met their deaths at the hand of the Melasa and the undead, or were slowly awaiting it locked away within the prisons.

Trivia

 * The Malaise could have possibly been influenced by the Black Plague.
 * Bupleurum is directly referenced as a method of treatment.
 * The Malasie appears to have had an affect on the appearance of the island.