Red pox

The red pox (or the Red Pox) is a disease created by Imperator Molok of Goria to sicken the encroaching orcs and force them from the ogre's lands, 400 years before the opening of the Dark Portal. Molok ordered the disease created after the ogres discovered legends about the Curse of Sethe, and they began experimenting with ways to induce a similar affliction among the orcs. Red pox is a highly contagious, wasting disease that kills many who are afflicted. The respected shaman Nelgarm learned from the elements that this was not a natural disease, but a weapon by the ogres. With so many orcs sick, the shaman of the clans resorted to asking the elements for aid against Goria. Even after Goria's destruction, the disease would never truly go away, returning in plagues every few generations.

Centuries later, Gul'dan used his magic to spread the worst outbreak in orcish history, killing hundreds in a matter of months. Many victims were infected during a Kosh'harg festival in Nagrand. In order to prevent the virulent plague from spreading to new victims, Ner'zhul urged Garad and the other infected orcs to remain in Nagrand. They constructed a new village, led by Garad before the red pox consumed him. After he died, Greatmother Geyah cared for them and named the village Garadar in his honor.

The pox's main symptoms were red pustules welling up on the skin, seeping a watery red fluid, and also the vomiting of blood. The disease was debilitating and affected many remaining "brown orcs." Those who suffered heavily perished within a matter of weeks.

When Kargath Bladefist arrived at an internment village he found many, including Garrosh Hellscream, son of Grom, affected by this disease. Nagrand had by that time become the home of the sick and weakened, but remained the most vibrant place on the otherwise dying world of Draenor (although it was not completely free of the corruption having suffered from long droughts). The Shattered Hand chieftain sneered at them for their "weakness" and made his way back to the battle.

The red pox would later be modified by the Shadow Council and melded with their fel energies. They released the modified disease on Shattrath City to fight its draenei defenders. The fel plague was loaded into crude bombs and lobbed at Shattrath. After the explosions, the mixture turned into a red mist. When its vapors reached them, the draenei immediately began choking. The draenei that survived the exposure of the mist were cut off from the Light and later transformed into the Broken and eventually the Lost One draenei. The most well-known survivors are Nobundo and Akama.

Speculation
It would appear that the red pox is part of the reason for the Mag'har and Nagrand escaping corruption; the other clans were likely to have given the afflicted a wide berth, thus they were never exposed to warlock magic. It should be noted that according to Rise of the Horde tribes living in Nagrand had not completely escaped corruption and that Nagrand itself suffered effects of the corruption but not nearly to the extent seen everywhere else on the planet.

The red pox might have made the orcs who caught it immune from the effects of fel magic, as well as the land. Though not clearly stated, Jorin Deadeye, Dranosh Saurfang, and most mag'har appearing in-game were probably also afflicted by the disease. The red pox has apparently been cured by the time of World of Warcraft: The Burning Crusade.