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You’ve heard people describe others as ‘parasites,’ but in this lesson, we’ll take a look at parasites from a biological perspective. What is a parasite? What do they do? What are examples of parasites? How do they impact humans? Read on to learn the answers.

Definition of a Parasite

Parasites are plants or animals that live on or in a host getting their nutrients from that host. A host is an organism that supports a parasite. Sometimes the host is harmed by the parasite, and sometimes the relationship is neutral. But the host never benefits from the arrangement. When the parasite does have a negative impact on the host, it doesn’t often kill the host directly, but the stressors that come with having parasites can kill.Ectoparasites are parasites that live outside the body.

In animals, they live on the skin and can cause itching and rashes. Endoparasites are parasites that live inside the body. For instance, they may live in the blood system, muscles, liver, brain, or digestive systems of animals.

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Parasites are not one-size-fits-all. There are several main categories of parasites that can affect animals and plants. Probably the most-studied parasites are the ones that affect humans. Broadly grouped, they include protozoa, helminths, and arthropods. More familiar, less mysterious names are lice, ticks, mites, bed bugs, flukes, and tapeworms.

Protozoa Parasites

Protozoa are single-celled organisms. They are usually found in food and water that is contaminated by animal waste.

Amoebic dysentery and giardia are examples of protozoa infections that invade the digestive system.In the case of giardia, the symptoms can include diarrhea, stomach cramps, weight loss, and dehydration. Some people have fewer symptoms, but severe infections cause dramatic symptoms.

Close-up view of the giardia parasite
Giardia parasite, close up view

Other protozoa are spread by a vector, an organism that transmits diseases, like a mosquito, and infect the blood or tissue. Malaria and African sleeping sickness are two protozoa parasites that might be familiar to you.

When an infected mosquito taps into your bloodstream to gather blood, they leave the parasites behind.One particularly interesting protozoa parasite is Toxoplasma gondii. It often infects cats, rodents, and people. Scientists have estimated that 1/3 to 1/2 of all people have the parasite.

Humans get it from eating infected sheep, pigs, cows, and from exposure to infected cat feces.What makes this parasite surprising is that it affects behavior. Infected rodents lose their fear of cat urine, which raises their chances of getting eaten by cats.

That passes the parasite to cats.In most people with Toxoplasma gondii, there are few symptoms of an infection, or perhaps they will have a short stint with flu-like symptoms. But in some people, it is thought to cause schizophrenia, suicides, and reduced reaction time.

Dividing Toxoplasma gondii parasites
Corpse flower (Rafflesia arnoldii) in bloom
Dwarf mistletoe in conifer tree
  • Parasites: plants or animals that live in or on other organisms
  • Host: those plants or animals parasites live off of
  • Ectoparasites: those parasites that live outside the body of the host
  • Endoparasites: refer to the parasites that exist within a host body
  • Protozoa: single-celled organisms
  • Helminths: worm-like parasites, including roundworms, tapeworms, and flukes
  • Arthropods: invertebrate animals with jointed appendages, segmented bodies, and exoskeletons (includes insects, crustaceans, arachnids)

Learning Outcomes

Once the lesson is over, students should be able to:

  • Define what parasites are
  • Explain what parasites do to hosts
  • Describe the types of parasites
  • Recall examples of parasitic diseases

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