From the Halo 2 Manual (Human Version): The basic infantry unit of the Covenant, Grunts are dangerous in groups but present little threat individually. Short, stocky, and relatively slow, they will often panic when faced with superior forces. However, if they are being led by an Elite, they will stand and fight.
From the Halo 2 Manual (Covenant Version): Stout and numerous, the Unggoy [Grunts] long have been the lifeblood of out Covenant's infantry. They will obey and order in the service of their duty and fight well with plasma pistol, turret, or in vehicles. Unggoy will as ever fight well with their comrades the Kig-yar [Jackals], and obey the directives of the Sanghelli [Elites].
The funny but occasionally dangerous Grunts, known as Unggoy to the Covenant, are back in Halo 2. However, the Grunts aren't simply cannon fodder this time, and should be feared in numbers. Because of the new AI in Halo 2, Grunts will be able to call for back up, as well as a give it. But as in Halo: Combat Evolved wipe a group of Grunts out as fast as you can, and you'll have no problems.
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