Helminth

A helminth is a parasitic worm. Unlike microbes, helminths are multicellular animals with organs, and the adults are usually big enough to see without a microscope. They are still studied in microbiology and parasitology because their eggs and larvae are microscopic, and these are the stages most often used to diagnose an infection, for example by finding eggs in a stool sample.

The helminths that infect people fall into a few groups. Nematodes, or roundworms, have a complete gut and include Ascaris, hookworms and pinworms. Flatworms include the flukes, or trematodes, such as Schistosoma, and the tapeworms, or cestodes, which attach by a head called a scolex and grow long chains of egg-filled segments. A smaller group, the thorny-headed worms, also infects humans. Adult helminths generally cannot multiply inside a person, so the number of worms builds up through repeated exposure. Many need an intermediate host, such as a snail, pig or cow, to complete their life cycle.

Soil-transmitted helminths, spread where human feces contaminate soil, are among the most common infections in the world and can cause anemia, poor nutrition and slowed growth in children. Regular deworming of high-risk groups is a major public health measure.