Your body is a habitat. It’s warm, wet, full of nutrients and unusually stable — the same conditions that make any environment worth moving into. And habitats get settled onto by parasites.

Most of us already accept this where bacteria are concerned; the gut microbiome has made that a comfortable idea. But bacteria aren’t the only organisms built to live on a person. Single-celled parasites do it too — organisms far more complex than bacteria, closer to miniature animals in how they behave. And so do actual animals: mites and worms, with legs and guts and life cycles of their own.

None of them require you to be sick, dirty or unlucky. They are routine in healthy people. So for most adults, the interesting question isn’t whether something else is living in you. It’s what. Here are three of the most common residents, and what each one reveals about the business of making a living inside another life .

Parasite 1: Toxoplasma gondii , The Parasite That Needs A Cat

Toxoplasma is a single-celled parasite with a peculiar constraint. It can infect nearly any warm-blooded animal, but it can only complete its sexual life cycle inside a member of the cat family. Everything else — livestock, rodents, us — is a waypoint.

In a waypoint host, the parasite settles into muscle and brain tissue as dormant cysts and, in someone with a healthy immune system, generally stays there quietly for life. Exposure varies enormously with region and diet: the Centers for Disease Control and Prevention estimates that around 11% of people in the United States 6 and older have been infected, while in some populations elsewhere the figure runs past 60%.

What draws biologists to it is what it appears to do to rodents. In a 2000 study published in Proceedings of the Royal Society B , infected rats lost their innate wariness of areas marked with cat odor — in some cases trading avoidance for something closer to attraction. From the parasite’s point of view, that is a transport strategy. Biologists call it host manipulation: a parasite shifting its host’s behavior in ways that serve the parasite’s transmission rather than the host’s survival.

Whether anything comparable happens in people is a separate question, and one where the popular story has run some distance ahead of the evidence. Associations between Toxoplasma exposure and human personality or risk-taking have been reported for years, but they remain contested — a 2016 study published in PLOS ONE , which followed a full birth cohort into adulthood, found little sign of the personality, psychiatric or impulse-control effects the popular version promises. The clinical concerns are narrower and much better defined: first infection during pregnancy, and reactivation in people who are seriously immunocompromised.

Parasite 2: Demodex folliculorum , The Mite On Your Face

Almost every adult reading this is carrying microscopic mites in their hair follicles, most densely on the face — the cheeks, the nose, the eyelashes. Carriage rises with age and is near-universal in older adults. They live head-down in the pore, feed on sebum, the oil your skin produces, and emerge at night to crawl across the skin in search of mates. They are acquired in early life, largely from close contact with parents, and they never leave. A follicle is a pocket in your skin, which puts these mites right on the boundary between on you and in you.

They are also, by the standards of their own phylum, falling apart in the most interesting way. Genome sequencing led by researchers at the University of Reading and Bangor University found one of the smallest gene counts of any related arthropod, alongside a drastic reduction in cell numbers — each leg segment operated by a single muscle cell.

This is what evolutionary biologists call reductive genome evolution, and it follows from a life of total dependence. An organism that will never live anywhere else, face any competition, or need to find a new host can afford to discard almost every tool it isn’t currently using. The mite has done exactly that.

A 2022 study published in Molecular Biology and Evolution argues that this places Demodex partway along a transition from parasite toward obligate symbiont — an organism so bound to its host that the distinction stops meaning much. That interpretation rests on a single sequenced genome and isn’t settled, but the direction of travel is clear enough.

For most people the mites cause nothing at all; they’ve historically been blamed for more skin trouble than the evidence supports, though a 2017 meta-analysis published in the Journal of the American Academy of Dermatology found that denser mite populations track with rosacea.

Parasite 3: Enterobius vermicularis , The Worm That Thrives In Clean Houses

Pinworm is the most common worm infection in the United States — overwhelmingly in school-age children, and typically in more than one member of a household at once. At night the female worm travels out of the intestine to deposit eggs on the surrounding skin. The itch that follows delivers eggs to fingers, fingernails and from there to mouths, bedding and clothing, where they can persist for weeks.

The biology that matters here is the direct life cycle: no intermediate host, no period maturing in warm soil. Many parasitic worms are effectively locked to particular climates and sanitation conditions by those requirements. Pinworm isn’t. It moves person to person indoors, which is why it has persisted in wealthy temperate countries long after the soil-transmitted worms disappeared from them. Infections are usually mild or symptomless, readily treatable and notoriously prone to coming straight back.

“Parasite” sounds like a permanent classification. It isn’t. It’s a position on a spectrum that runs from lethal exploitation through freeloading to genuine partnership, and organisms slide along it over evolutionary time — the face mite may be sliding right now.

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