The invisible hitchhikers that thrive at the expense of their hosts

Science · Published 23 October 2026 Swipe more like this →

In short: They live on, in, or with us, deriving nutrition without offering any benefit in return. While most parasites avoid killing their hosts to ensure their own survival, these organisms can carry life-threatening diseases, ranging from microscopic single-celals to worms over a meter long.

Parasites are broadly categorized into three classes that infect humans: protozoa, helminths, and ectoparasites. Single-celled, microscopic life forms known as protozoa may exist independently or as parasites. Because they can multiply within a human host, a single organism can potentially trigger a serious infection. These are often classified by how they move, such as amoebas using 'false feet' (pseudopods) or ciliates using hair-like structures called cilia. Some, like the amoeba Entamoeba histolytica, are known to cause dysentery.

Helminths, or parasitic worms, are multicellular and often visible to the naked eye in their adult stages. They can range in size from just over 1 millimeter to more than 1 meter in length. Common groups include flatworms, such as tapeworms (cestodes) and flukes (trematodes), as well as roundworms (nematodes). While adult helminths typically reside in the gastrointestinal tract, their larval stages can infect various body tissues. Notably, unlike protozoa, adult helminths cannot multiply within the human host.

Ectoparasites live on the exterior of the host. This group includes organisms like ticks, fleas, lice, and mites. Many of these act as vectors—living organisms that carry pathogens between animals and humans, often through blood. For example, mites are tiny arachnids, some smaller than 1 millimeter, while lice are flat insects that travel via close contact. While some arthropods like mosquitoes are technically dependent on blood meals, the term ectoparasite is often used more narrowly for organisms that attach to or burrow into the skin for extended periods.

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