Pentapox was the name of a pseudo-illness invented by Katara and Sokka to prevent Fire Nation guards from discovering that Team Avatar did not belong in New Ozai. This alleged "disease" was then used to aid the Omashu Resistance's escape from the city.
While in the Fire Nation-occupied city of Omashu, Aang, Katara, and Sokka found themselves cornered by Fire Nation guards, one of whom noticed Sokka's spots from the purple pentapuses that the gang had encountered previously. In order to keep their identities concealed, Katara said that Sokka was suffering from pentapox, a fictitious disease that she invented on the spot. Her brother played along by pretending to suffer from serious physical pain. Concerned about their own safety, the Fire Nation guards fled in terror.
To escape the city of Omashu, Sokka concocted a plan that involved the use of the disease: a pentapox plague. The soldiers of the resistance handed out pentapuses, which the citizens used to cover themselves with suction markings; they proceeded to walk around the city in a manner similar to zombies, moaning that they were gravely ill. On the governor's orders, the citizens were driven out of the gates to rid the city of the plague.
Katara described the epidemic as fatal and highly contagious. A person who had pentapox could be easily identified by the numerous purple markings all over his or her body. The marks were created with the use of a purple pentapus, which clung to human skin and left behind markings from its tentacles upon removal. The "infected" also adopted a lurching walk, slack expression, and loud moan, making them resemble a zombie horde.
- Toph preempted Sokka when he was about to explain his plan to take over enemy ships in "The Bridge", assuring that he would not tell his "pentapox story" again.
- The alleged indicators of pentapox resemble those of the real world disease chickenpox, an illness that also has distinct red markings as a symptom.
- The incidence in which one of the guards mistakenly recalled his cousin's death from pentapox mirrors the psychological process of suggestion, which is known to cause the formation of false memories.