parasite
Meaning of parasite: Many have life cycles passing through more than one species, so breaking the cycle can matter as much as treating the person.
Definition of parasite
A parasite is an organism that lives on or inside another organism, the host, and takes nourishment from it while giving nothing in return and often causing harm. Those relevant to human health fall into three broad groups: single-celled protozoa, multicellular worms known as helminths, and external parasites such as lice, mites and ticks.
What are the main groups?
| Group | What they are | Illnesses they cause |
|---|---|---|
| Protozoa | Single-celled organisms able to multiply within the host | Malaria, giardiasis, amebic dysentery, toxoplasmosis |
| Helminths | Multicellular worms, generally unable to multiply inside the host | Roundworm, tapeworm, hookworm, schistosomiasis |
| Ectoparasites | Organisms living on the skin surface | Lice, scabies, tick infestation |
The distinction between the first two groups matters practically. A single protozoan can multiply into an overwhelming infection from a very small initial exposure. Worms usually cannot increase their numbers inside a person, so the severity of illness depends largely on how many were taken in and how often exposure is repeated.
How do they cause illness?
Damage arrives by several routes. Some parasites destroy cells directly, as the malaria parasite does to red blood cells. Some consume nutrients meant for the host, producing anemia or, in children, impaired growth. Some obstruct, since a mass of worms can block the intestine or a bile duct. Some damage tissue mechanically as they migrate through it. And some cause harm mainly through the reaction they provoke, because the immune system's response to a parasite it cannot clear inflames and eventually scars the surrounding tissue. An allergic response is common as well, and a raised eosinophil count in a blood sample is a familiar clue pointing toward worm infection.
How are they transmitted and identified?
Routes include contaminated water or food, undercooked meat or fish, insect bites, contact with soil, and direct person-to-person contact. Many have life cycles running through more than one species, with an insect or an animal serving as an intermediate stage, which is why control measures often target the insect or the animal reservoir rather than only the affected person. Identification usually means finding the organism itself or its eggs in stool, blood, urine or a tissue sample, rather than culturing it the way a bacterium would be cultured. Antibody tests exist for some, and imaging finds those that form cysts or masses inside organs.
Used in a sentence
Microscopic examination of the stool sample identified ova consistent with an intestinal parasite.
Good to know
A parasite differs from other infectious agents such as bacteria and viruses in being a distinct organism with its own life cycle, which is why parasitic infections are often identified by direct examination for the organism rather than by the methods used for bacterial or viral illness.
Word origin
Where “parasite” comes from
Formed from Greek parasitos, from para-, meaning beside, and sitos, meaning food, an early term for one who ate at another's table without contributing.