These are assumed to be humans in an advanced state of infection. They appear disgustingly bloated and deformed; their bodies almost completely overtaken with strange metallic tumours.
In terms of behaviour, they are almost exactly the same as their early infected counterparts- clumsy and uncreative. They are, however, far more difficult to kill due to increased health, often getting up several times after being shot down. In some cases, if damaged enough, an infected's body will bloat and retract uncontrollably. This means that it is going to explode, so get away from it quickly. Being caught in the blast radius will cause significant damage to the player. On the other hand, causing enough damage will cause the infected to sometimes glow red allowing you to perform a stylish finisher. This can be either slicing off the right arm with the Glaive, destroying the creature's head with its own melee weapon or, if executed from behind, a neck-breaking headlock. Nasty!