"reproducing with multiple life cycles."
As AvPG's resident lab tech/parasitologist there are some parasites, specifically plasmodium species(cause of malaria) that have an asexual and sexual stage of reproduction depending on the situation. I
As far as nematodes go, humans are the definitive host and some organisms produce either fertilized eggs or larvae that may be infective to a new host.
Stay tuned for more fun facts with AvPGs resident lab tech/parasitologist.