Leeches are not among the most common of goldfish parasites, but nevertheless, they are rather nasty, and easily introduced into an existing tank.
Leeches live by latching onto a living host (such as a goldfish) and feasting on the blood of the host, before detaching and dropping away when they are full, and then repeating the process every time that they are hungry.
Understandably, leeches can pose a health hazard to goldfish, both through blood loss and through the risk of infection to the abrasions left behind after they detach from the skin.