Hookworm
The hookworm is a parasitic roundworm that infects the small intestine of humans. They are usually from 1/2 inch to 1 inch long. They can be found and are widespread in countries with tropical or subtropical climates. Their eggs are passed through the feces of humans, and they thrive in the soil in which these feces may be placed. Hookworms do not have a formal respiratory system, but rather breathe through diffusion. Diffusion is a process in which oxygen enters through the hookworm's skin while carbon dioxide and other waste products exit from it.