The red pox is a disease that ravaged many orcs in the years following the rise of the Horde. 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". Its origin is unknown.
When Kargath Bladefist arrived at a quarantined village in Nagrand — likely the modern town of Garadar — 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.
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 affects of the corruption but not nearly to the extent seen everywhere else on the planet.
It might be possible that the red pox might have inoculated the orcs who caught it from the effects of fel magic, as well as the land. Though not clearly stated, Jorin Deadeye, Saurfang the Younger 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.