The Queen's Lady Plague refers to an outbreak of firepox in Ketterdam about seven years before the events of Six of Crows. It was named after a ship, the Queen's Lady, which was believed to have brought the disease to the city. Hundreds perished, including Kaz's brother, Jordie Rietveld. Kaz himself contracted the disease, but survived the fever, waking to find himself floating on the Reaper's Barge amidst countless corpses.
Being situated on an island that had suffered outbreaks of plague for at least a hundred years, burial was prohibited within the city limits of Ketterdam in order to prevent the spread of disease from decimating the population. Instead, all bodies must be cremated, or else sent outside the city for burial. For those times when the spread of plague made itself evident, the plague sirens sounded to signal all citizens to return to their homes, and the officers of the stadwatch to report to their designated stations around the city. Only the sickboats, bodymen, and mediks were allowed to move freely about the city during an outbreak.
In Crooked Kingdom, Nina uses her new ability to manipulate decaying cell matter in order to mimic the symptoms of plague. She directs the black cells – taken from a regular corpse – to spread over the skin of patrons at the clubs run by Pekka Rollins and his gang, the Dime Lions. This false plague sends the city into a frenzy, destroying the reputation of the Dime Lions and Pekka Rollins in the process. After the Dime Lions clubs are determined to be sources of the outbreak, their business is shut down, cutting off their main sources of revenue. Nina also targets the Menagerie, as per Kaz's instructions.
- The Queen's Lady Plague was probably inspired by the Black Death that swept Europe in the Middle Ages, in that it caused skin to blacken with necrosis and it killed 30–60% of the population.