The Gray Plague, also known as "KoLera", "KoLeprosy", "KoLd", "Jicken Pox", and "Blanket AIDS", was a sickness that infected the Kingdom for about three weeks during October and November of 2005. The source of the disease was suspected to be comfy blankets, which were given out by Cristobal Colon (#1492) on Columbus Day, October 11, 2005. Though the sickness vectors were unknown, it was suspected that players could contract the disease by resting at their campground with a blanket in their inventory, or by chatting with (or receiving kmail from) infected players.
The disease manifested itself in chat and kmail. The first stage of the disease was the continually more frequent appearance of a gray -cough- after a line in chat. On October 14, 2005 (after rollover), those who had been infected with the Gray Plague on the 11th had a dark grey chat text color instead of black. This effect is very subtle, but was verified by Crapstorm, a screenshot, and GIMP. The hex code for the grey text color is 333333. Some members of the community even went so far as to call the disease CS33 in honor of Crapstorm's work. On the 16th, an even lighter grey text – 666666 – was noticed in chat. On the 17th, the text of some people had turned an even lighter shade of gray, the same color as the -cough-.
When wearing rose-colored glasses obtained from the Suspicious-Looking Guy, a player's own text no longer appeared affected by the plague; however, this effect was limited to the player. Other infected players would still show gray text, and the infected player's gray text would still be visible to others. Doc Galaktik had a temporary Miracle Cure available if you brought him the right item, but players would relapse if they were exposed to the disease again.
Eventually the council commissioned all adventurers to help create a plague vaccine.
Throughout the epidemic, a section of the KoL homepage was dedicated to listing how many people were infected by the Plague. On November 5, 2005, the tally went down to zero, marking the end of the Gray Plague era.
Players who were infected and eventually cured via the plague vaccine have the statement "You have been inoculated against the Grey Plague" in the "Other Accomplishments" section of their Quest Log.
- The name of the disease is a parody of a real pandemic called the Black Plague.
- Possible reference to the game Deus Ex, where a manufactured plague called the Gray Death threatened the world.
- Possible Reference to Kevan's Urban Dead another Online "low-tech" Multiplayer game involving a zombie infestation in the fictional, quarantined city of Malton. Zombie Players may infect Human Players who then must be healed before they become zombies themselves. If they are not healed, they lose HP, and when they die, they rise as Zombies. Many Players elect to stay Zombies while others seek immediate "revivification".
- Possible reference to the Corrupted Blood plague that broke out in World of Warcraft in September 2005, killing hundreds of characters.
- The source of the plague being comfy blankets references Jeffrey Amherst's plan to infect American Indians with smallpox by giving them infected blankets.
- The plague is later referenced in The Worm Wood and by the the boss of the Zombie Slayer Special challenge path.