The swarm is a random event. It attacks a player, and the player cannot attack it back. It hits for either one or two points of damage. This may not seem like much damage, but when it all adds up, it can be lethal. All the player needs to do is walk or run away for a short distance, and then the swarm will stop attacking. If they don't, they will eventually be killed by the swarm.
They can still be found on the docks of the Piscarilius House, often near the barrels of fresh fish used in gaining favor. They are harmless and will not attack the player, but will follow them within a certain range similar to the chair in Draynor manor.