There was a time without Virus or Vaccine. The digimon just went about their lives, and the circle of life went on, not worrying about destruction or darkness. There were still battles. After all, digimon are made for battle. But there were no wars. It was peaceful. Natural. Everything was in perfect harmony.

And then he arrived.

Nobody knew his name or what he looked like, or even if he was in fact a physical being. But wherever he went, devastation followed. Dark clouds covered the wastes of the lands he had corrupted. If digimon were too weak, they were simply erased. If they proved to be strong, they were changed. Their head were filled with thoughts of violence, greed and cruelty. They enjoyed giving torment to others. All that did not share this view were their enemies, and those that did were still not to be trusted. The virus digimon were born.

Of course, there must always be balance. The four gaurdians of the digital world had always watched, but they had never acted. Now that a great threat had arrived, the four had to create something to counter it. Using their combine power, they changed certain digimon species into vaccine. These new digimon were to protect the order of the digital world and fight to wipe out the unholy virus digimon that had been spawned by this great darkness.

Even these new digimon were not immune to the effects of the Great Dark One, as his followers had now begun to call him. A number of vaccine digimon were changed into demonic warriors, but at the same time some virus digimon decided to go against their ways and fight to preserve order. Soon, a mass war erupted between the two sides over control for the digital world. Huge armies of digimon were formed, and they went to battle. While in the air mighty Kabuterimon fought against vicious Kuwagamon, on the ground Leomon and Ogremon engaged in a mighty struggle. There were many more digimon, but with those particular ones, endless rivalries were formed.

Eventually, the four sacrificed some of their own power to seal the Great Dark One. He was banished from the digital world forever. But the damage had been done, and the virus and vaccine digimon continued to fight for the rest of time. The four gaurdians created an order of beings similar to the digimon to help stop the virus digimon becoming too dangerous, who would work in secret rather than engage in battles. Less digimon would be lost this way.

After a while, evolution could continue without the influence of higher beings, and more natural digimon evolved. The two sides died down, and the virus and vaccine became a normal part of digimon evolution, along with the data digimon. Still leaders rose out, often Ultimates or Megas who felt a need to prove their power. Territories and borders appeared, but there was no longer mass destruction. It was assumed that this era of peace would last forever.

Nothing lasts forever.