An unknown mutation has caused these normally peaceful creatures to become much more dangerous. Adult worms are capable of transporting infested creatures, up to and including agrons in size, across the surface of the moon, done by forming a tube through the worm's body. This results in an unlimited number of zombies wherever these Nydus Worms surface, easily overrunning the marines unless these worms are killed.
Smaller worms usually live passively underground, not big enough to transport large infested. Occasionally one of these smaller Nydus Worms will emerge and unleash a swarm of parasites. Certain creatures such as Menoetes are capable of awakening these worms and directing them towards the marines. These worms will severely damage marines that are foolish enough to stand above them as they erupt. The emerging worm will damage the leg augmentations of the marines, causing them to be crippled.
An adult Nydus Worm is a transport highway for the infested, enabling them to travel from all over the moon to the location of the worm's head. This allows the Nydus Worm to bring multitudes of creatures.
A Nydus Worm lines its transport tubes with creep, accelerating movement of creatures within it. This creep is also spread around the worm where it erupts. Marines can often tell where a Nydus Worm is without seeing it by the creep it spreads. However, engaging a Nydus Worm is also much more dangerous at close quarters, as the creep makes it harder to outrun the infested spawned from the worm.