"This infected body is very weak and slow."
These slow zombies litter the map and stand around aimlessly, wandering slowly closer to the command center as the game goes on. They have very poor noise-detection, making them rather inactive even when the colony generates a medium amount of noise nearby.
In small groups (between 4 and 7) they can overwhelm a Ranger defending the perimeter if the Commander doesn't micro-manage the ranger. Due to their tendency to so slowly walk into range of a building, it is easy to miss them moving into the corner of the colony and infecting a building.