Blizzard like having world events in World of Warcraft.  There are regular events such as the Darkmoon Faire, seasonal events such as Hallows End and for special occasions there are events such as the Scourge Invasion currently underway.

The last time the Scourge Invasion made an appearance, it was for the release of the 40 man raiding instance Naxxramas, which was the last raiding instance in the original game before the first expansion The Burning Crusade was released.  This time its different, Naxxramas has disappeared from Azeroth and now lurks over the frozen wastes of Dragonblight in Northrend, where Dalaran once was there is only a crater, but the Scourge are back in Azeroth and have started their attacks.

First it started off with a mysterious plague turning unsuspecting players into ghouls, then the addition of necropolis' floating around certain zones with scourge summoning circles appearing nearby and now abominations and frostwyrms are attacking all of the major alliance and horde cities.  The city leaders are involved in this latest, and even leads to Garrosh Hellscream and Thrall fighting against each other in Orgrimmar.

No doubt there will be more to come in the next couple of days before the continent of Northrend, home to the Lich King himself becomes available for play with the second expansion for World of Warcraft - Wrath of the Lich King that is due out on Friday.

MMO-Champion has more coverage of the Scourge Invasion event as it happens here.