The Fleas are characters in the episode "Flea or Die!".
The fleas are parasites that used to reside on the Rat. When CatDog was seen by an individual flea they transfer to Cat. As a result of this Cat and Dog visit Rancid for treatment, in this case they wear a flea belt.
One night the fleas party in Cat causing him to be awakened. In the morning Cat fights for the belt from Dog causing fleas to move from brother to brother back and forth to whoever does not wear the belt. Cat would get the belt but when the fleas made Dog dying he decides to revisit Rancid for the flea bath. As Cat submerges himself in the flea bath all fleas in him are killed.
In the ending of the episode another flea swarm wears a belt as belts are a clothing trend.