parasites are contageous, but only if someone has a weaken digestive/immune system that will allow them to proliferate.
People think you can take few rounds of antiparasitic pills or herbs, and get rid of them. It is much more complex than that. If you have parasites in amounts that produce debilitating symptoms, you can assume your gut flora, liver, digestive juices are all compromised to some extent. Only a healthy Diet and adequate measures to rebuild the digestive system can get rid of them permanently and prevent a recontamination.