One could say that it started out simply enough.
One could also say that it happened with the best of intentions.
One could also blame it all on a passel of kids and their bison and mutter obscenities under their breath.
One could blame the poor sanitation systems that ran underneath the city.
One could also, quite conviently blame the Avatar. If he hadn't needed to return to Omashu to find and Earthbending instructor then none of this would have happened. It was also quite safe to say that it was Sokka's fault. It was, after all, his plan.
He was also the first casualty.
War is a cruel mistress and picks her victims from every walk of life.
Our story begins with three youngsters walking through a sewer to sneak into the Fire Nation controlled City of Omashu. Aang had returned from a failed attempt to learn Firebending to learn how to move rocks. At least that's what he remembers Jeong-Jeong telling him. He was having too much fun laughing at Zhao to really pay attention to the old man. Ah, good times watching the Admiral burn down his own boats. It still brought a tear to his eye.
When they had made it out of the tunnels and into the light, Sokka discovered that he was covered with tiny pink leech things with tentacles. Pentapuses, Aang explained, gently pulling the suckers off. They live in the sewers. They also left multiple red marks in suspicious patterns all over Sokka's body. Disgusting, but not helpful.
Well, it became disgusting and very helpful. It helped evacuate the entire city. Everyone escaped and was relocated and not a single person was injured. It was, it the GAang's opinion a supremely excellent day. So the left Omashu and went off looking for the master the Bumi had described. What they didn't know was what they started.
Viruses are capable of living on air molecules for a few days after a sneeze or cough. Illness spread from person to person by shaking hands, breathing the same air, kissing, drinking out of the same cup and the list goes on. Most illnesses are simple and are reasonable dealt with. Even the most severe illness if caught in time is merely trivial. Most of the time it didn't matter if you had a Waterbender with you. If you were lucky they'd just bend the illness right out of you and away you went, as right as rain.
Unfortunately, the universe just loves proving Sokka wrong.
Sokka had always loved meat. It was his life. He enjoyed hunting and fighting because it was good practice for getting meat. There were so many things that he loved about meat. However, a few weeks after the evacuation of Omashu, Sokka noticed that his thoughts about meat were getting steadily darker. Like taking the sleeping Momo and shoving a stake through him and then roasting him over the fire pit.
The first time this thought occurred to him, he walked away from the camp and threw up his dinner.
The second time he went for a walk and did not come back. When Momo found Sokka's broken form at the bottom of the gorge, his blood was black and oozy like pus. Momo also did not return to camp.
The virus moved steadily and quickly throughout the Earth Kingdom, many suppressing the symptoms until it was too late. By the time they realized what had happened, they shut down every port but the epidemic was now world wide. It wiped out the Earth Kingdom. In Ba Sing Se the Dai Lee shattered the great walls that surrounded the City. The forest once protected by the Freedom Fighters was now haunted with empty bones and ghost blades.
The Water Tribes were perhaps, the most resistant having blocked all contact from the outside world. Any who were caught with the virus were healed repeatedly or simply disposed of. The Southern Tribe had no cases and her northern sister only lost a handful of souls.
The Fire Nation was a glass of water filled to the middle, half full and half empty. The naturally high body temperatures killed off the viral invaders for most of the citizens. Others were not so lucky. A good half of the Fire Nation vanished up in smoke that darkened the horizon for weeks. The virus was brought back by navy vessels, merchant ships and by a small elite team from the capital.
On a small island off the coast of the Earth Kingdom, the saddle of the Avatar's bison washed up onto the shore.
After the situation had become secure, an old man sat back onto the throne of the Fire Nation. When asked later what had become of the world he stared at the Pai Sho board before him. He spoke softly, saying that the world paid for the sins of the Fire Nation. Perhaps with the burning of the world, like the burning of grass, new ground might be laid down and a new world planted. It was a blessing in disguise, a ray of sun behind a storm cloud. The Avatar did save us in the end. It was just not to our liking.
"But, sir," a young page asked, head tilted in curiosity. "You travelled throughout the Earth Kingdom – and the world extensively. You must have come in contact with the disease. Why did you, of the entire royal family, not succumb?"
Iroh lifted his tea cup in a humble salute.
"I drink tea for a reason young man."