Lots of different species, though, so be careful, not all nematodes are beneficial; some are not. Many nematode species are parasitic. While the vast majority of nematodes are "free-living", a significant portion, particularly among larger species, have evolved to be parasitic, impacting both plants and animals. Nematodes can infect humans and cause various diseases. These parasitic worms are a significant public health concern, with billions of people infected globally. They can be transmitted through various routes, including skin penetration, ingestion, and insect vectors.
Not something you can easily remove once you add, beneficial nematodes can effectively eliminate fungus gnat infestations. Once they have consumed all the fungus gnat larvae in the soil, the nematodes will naturally die off due to a lack of food. Just make sure you are doing a little homework prior and don't just dump in any old farmer joes crap. Good luck.