What is the common name for parasitic worms?

Parasitic worms are more commonly known as Helminths. These are large, multicellular organisms that are visible with the naked eye. 

They are parasitic because they feed on a living host to gain nourishment and protection, while often resulting in illness of the host.  Helminths are one of the leading cause of morbidity in the developing world, with over two billion people affected - almost one third of the worlds population.