Pentapox

Pentapox was the name of a fake illness invented by Katara and Sokka to distract Fire Nation guards from discovering that Team Avatar did not belong in Omashu. This "disease" was then used to aid the Omashu Resistance's escape from the city. It is possible that Pentapox may have been a real disease, as one of the guards seemingly knew about the illness, referring to his cousin Chang, although the symptoms he identified were vastly different.

Description
Katara described the epidemic as fatal and highly contagious. A person who had Pentapox could be easily be identified by the numerous purple markings all over his or her body. The marks were created with the use of a Purple Pentapus; this animal clung to human skin, leaving behind markings from its tentacles upon its removal. The "infected" also adopted a lurching walk, slack expression, and loud moan, making them resemble a zombie horde.

History
While in the Fire Nation-occupied city of Omashu, Aang, Katara, and Sokka were cornered by Fire Nation guards, one of whom noticed Sokka's spots from the Purple Pentapus that the gang had encountered previously. In order to conceal their identity, Katara lies by saying that Sokka was ill with Pentapox, and Sokka helps his sister by pretending to have serious physical pain. Concerned about their own safety, the Fire Nation guards flee in terror.

To escape the city of Omashu, Sokka utilized a plan that involved the use of the disease. He planned to stage a fake Pentapox Plague; the citizens of Omashu, who were displeased with its recent conquest, walked around the city as if they were gravely ill. On the Governor's orders, the citizens were driven out of the city. When word of this reached Princess Azula, she berated the Governor for damaging the Fire Nation's control over the city.