Worms have been living inside the human body since Homo sapiens have been around. About half the world's population (over 3 billion people) are in infected with at least one of the three worms forming what Columbia University parasitologist Dickson Despommier calls the "unholy trinity"—large roundworm, hookworm and whipworm. Most of those afflicted live in developing countries, where there is not enough clean drinking water or effective sanitation systems to keep infected feces from contaminating food and water, and where human excrement is used to fertilize crops. The most prolific parasitic worm in the U.S. and European Union: the pinworm, which is most common during childhood.

Despommier kindly lent ScientificAmerican.com images of the unholy trinity and their worm relatives, which also appear in his book Parasitic Diseases. As you browse through these slides, please remember that all of these infections are treatable.

Slide Show: Worms 'N Us

Note (2/6/09): Two of our readers, froynlaven and Nathaniel, pointed out that this slide show might need a warning. So be advised, some of the images are graphic depictions of medical conditions.