They can only be found during Raid Mode. They appear as large, bright orange puffer fish. Their only attack is jumping at the player, dealing minimal damage with their spikes.. When damaged themselves, they try to get away by flopping around on the floor.
They have extremely low health, but incredible defense - so much that it renders most weapon attacks to a minimal damage.
During Raid Mode, they tend to appear in optional areas. Upon defeat they will reward the player with a Weapon Case, which has a high probability of containing a stronger weapon.
Since they are only found in Raid Mode, it is unknown as to what viral process created them, if they are even a canonical enemy, or if they are just simply created as a gameplay mechanic unique to Raid Mode.
Interestingly, they are regarded as Ghiozzo when killed (this can be observed by killing them while going for the Ghiozzos' killing mission).
The Infected Blowfish's high defense means that killing one can deplete your ammo reserves. The shock grenade, however, does significant damage. With a well placed throw the blowfish can be killed with a single shock grenade. As they aren't particularly dangerous using the knife to attack is also a good option to conserve ammo.