Parasitic worms are sequential hermaphrodites and reproduce depending on the species of worm, either with the presence of a male and female worm, joining sperm and eggs, producing fertile eggs, such as hookworms, or by breaking off segments that contain both male and female sex organs that are able to produce fertile eggs without the presence of a male or female (e.g., tapeworms).

All worm offspring are the passed on through poorly-cooked meat, especially pork, wild fish, and beef, contaminated water, feces and mosquitoes. However, it is estimated[who?] that 40 million Americans are infected with the most common roundworm, the pinworm.

Worm eggs or larvae or even adults enter the human body through the mouth, anus, nose, or skin, with most species attaching themselves to the intestinal tract. With the presence of digestive enzymes, worm egg shells are dissolved, releasing a brand-new worm; unlike its egg shell, the parasitic worm is protected from the body’s powerful digestive enzymes by producing a protective keratin layer.