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Infection and immunity
Infection and immunity

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3.4  Introducing parasites and protists

The term parasite refers to an organism that lives in or on the body of another organism (its host) and benefits at the host’s expense, so strictly speaking all pathogens are ‘parasites’. However, in discussions of human infectious diseases, the term is restricted to:

  • multicellular parasitic worms
  • pathogenic protists: single-celled organisms with similar cells to those of animals (multicellular protists exist, but none are human pathogens).

In 2013, the World Health Assembly identified 17 so-called ‘Neglected Tropical Diseases’ affecting over one billion people; it is notable that eight are caused by parasites and three by protists. Figure 2 shows sites on the surface or inside the human body that parasites and protists can infect. Click or tap on each of the labels in Figure 2 to reveal a brief description in a box below the diagram of the disease states the pathogen causes and the site(s) it occupies in the body.

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Figure 2  Some parasites and pathogenic protists, the diseases they cause, and and the sites they infect in the human body.
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This is a diagram of the human body annotated with names of a number of parasites and pathogenic protists to show the organs and tissues that they infect. These are: head louse – scalp; tapeworm cysts – brain; Toxocara – brain, eye, lung and liver; filiarial worm – eye and lymph nodes (e.g. those under the arm); Plasmodium – brain, bloodstream and liver; Trypanosoma – heart and bloodstream; liver fluke – liver and gall bladder; Entamoeba histolytica – liver and intestines; Schistosoma – liver, intestines and bladder; tapeworm – intestines; hookworm – intestines; Trichomonas – bladder; and body louse, scabies and ticks – skin surface. Clicking on the labels reveals some details about each parasite or pathogen. These read:

  • head louse causes irritation; site: head hair
  • tapeworm cysts cause epileptic seizures; site: brain
  • Toxocara [tox-oh-kah-rah] adult can cause blindness and liver damage; sites: eyes, brain, muscle and liver
  • Plasmodium [plaz-moh-dee-um] causes malaria; sites: red blood cells, liver and brain
  • filarial [fill-air-ee-al] worm can cause fever, elephantiasis and river blindness; sites: lymphatic system and eyes
  • Trypanosoma [trip-pan-oh-soh-mah] cause sleeping sickness; site: blood
  • liver fluke cause chronic liver disease; sites: liver and gall bladder
  • Schistosoma [shist-oh-soh-mah] cause bilharzia; sites: intestine, liver and bladder
  • Entamoeba histolytica [en-tah-mee-bah hist-oh-lit-ik-ah] cause amoebic dysentery; sites: liver and large intestine
  • hookworm can cause anaemia and protein deficiency; site: large intestine
  • tapeworm can cause anaemia and malnutrition; site: intestines
  • Trichomonas [trik-oh-moh-nas] cause irritation and discharge; site: reproductive tract
  • body louse can transmit typhus and trench fever; site: in clothing and on skin
  • ticks cause irritation, can transmit typhus and various fevers; site: on skin, feed by sucking blood
  • scabies cause severe irritation; site: on top of skin, males dig burrows under skin
Figure 2  Some parasites and pathogenic protists, the diseases they cause, and and the sites they infect in the human body.
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