As a result of being administered the t-Abyss virus, the Farfarello came to have the ability to mimic their surroundings and become transparent similar to a group of aquatic organisms. However, because they were exposed to multiple viruses (both the t-Virus and t-Abyss), they became so aggressive that they were even more difficult to control than a regular Hunter.[excerpt 1]
↑From the Resident Evil.net Creature info page: Farfarello are created by improving Hunters with the administration of the t-Abyss virus, which enables them to turn invisible and blend into the surrounding environment, an ability usually found only in a number of aquatic organisms. The presence of multiple viruses has also increased their aggression however, and they are less controllable as a result.