The cockroach is a common bug found throughout the USA and as such where there have been
t-Virus outbreaks the cockroaches have been afflicted with the effects of the virus which mutates them to a small degree.
Essentially they grow in size to be of a similar size to a common
Rat, but they do not mutate particularly large nor do they show any outward symptoms of the
t-Virus - so it may be that their physiology is just too simple for the virus to make any substantial changes.
However, there have been occasions where swarms of aggressive cockroaches have attacked small animals such as rats - but this was seen as a fairly positive thing as they reduced the
Rat population of Raccoon City to almost zero. But once the
Rat population dwindled enough, the cockroaches would then turn to humans for sustenance - although reports of this have been rare.
Unlike the
t-Virus, the more powerful
Uroboros Virus was able to mutate an infected cockroach to such a degree that it resembled an enormous monster with various kinds of insect features such as barbed arms and legs, scythes on the end of its limbs and extremely sharp teeth. It was also capable of releasing a poisonous gas cloud - although the creature itself was given it's own name - the
Reaper.
