Roundworms eat algae, bacteria, fungi and protozoans. The majority of roundworms are carnivorous, feeding off plants, humans and animals. Certain species are filter feeders, which are animals that separate food particles from water.
Roundworms range in size from microscopic to several meters long. Their long, tube-shaped bodies contain complete digestive systems but no circulatory or respiratory systems. Lifecycles vary depending on the species, but parasitic varieties can move between multiple locations within the host's body. Roundworm infections occur primarily from eating uncooked meat containing larvae. They can also enter the body through unprotected cuts or through transfer from bloodsucking insects.