The plague built up through several phases:

WotLK Zombie Invasion

Phase 1

Suspicious Crates in Booty could be looted, immediately giving the player the plague. You could also get the WotLK zombie plague (You’re Infected!) by standing in a zombie’s disease cloud, or by being hit by a zombie.

Once you are infected you can be cured by any class who can cure diseases (Priest, Paladin, Shaman), or by Argent Healers that are now located throughout the world.

You have 10 minutes before the plague turns you into a zombie. You can also immediately be turned into a zombie if you are killed by a zombie.

Once you transform into a zombie you gain several abilities:

Beckon Groan – Brings up to 4 zombies that are not currently in combat to you.

Lurch – AoE buff that causes all zombies in the area to move more quickly, and removes any crowd-control or movement impairing effects on them.

Mangle -Reduces your target’s armor by 3% and increases your next attack damage by 20.

Retch – A targetable AoE that heals all zombie in the area, and slows all humanoids.

Zombie Explosion – A kamikaze ability, destroying you and infecting non-zombies in the area with the plague.

When you are a zombie your health decays over time, however you can gain health back by attacking living characters and NPCs. You can also use your Retch ability to regenerate lost health.

Players infected with the zombie plague also all speak [Zombie]. This temporarily allowed cross-faction communication between Alliance and Horde players.

Phase 2

WotLK Zombie Invasion Plagued Roach

The Suspicious Crates spawned in all major cities, along with Plagued Roaches. Killing the plagued roaches or looting the crates immediately gives you the zombie disease.

Phase 3

In the third phase “Plagued Residents” (NPC zombies) began spawning outside of major cities. The plague timer was reduced from 10 minutes to 5 minutes, and the radius of the zombie explosion ability was greatly increased.

Phase 4

The infection debuff timer was reduced again. Now it is 2 minutes, down from 5 minutes. The Naxxaramas Scourge Invasion event has begun again. There are now invasions of Scourge all across Azeroth.

All of the rare bosses from the previous Scourge event are again located in their respective instances.

• Lady Falther’ess in Razorfen Downs
• Sever in Shadowfang Keep
• Scorn in Scarlet Monastery‘s Graveyard
• Revanchion in Dire Maul (West)
• Lord Blackwood in Scholomance
• Balzaphon in Stratholme

All of these bosses have unique loot tables with rare items.

A new boss was added to Karazhan, Prince Tenris Mirkblood. There is a quest to kill him located at Light’s Hope Chapel in the Eastern Plague Lands.

Phase 5

The zombie disease debuff timer was reduced to one minute. Nearly all of the Argent Healers have disappeared, leaving the plague and zombies to overrun every city in the game.

Grand Apothecary Putress was introduced to Shattrath City. He requires you to get a sample of this new Scourge plague which has a slightly different questline ending for each race. For the Forsaken you take a sample to the Royal Apothecary Society in Undercity for further study.

Phase 6 – Finale

The zombie plague ended. The Scourge invasion and Tenris Mirkblood will remain for a week. Blizzard says the WotLK opening event is not over, but the zombie plague is… for now.

For more information on the WotLK plague from Blizzard you can check the following thread:

Official WotLK Zombie Invasion FAQ