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Infection and immunity
Infection and immunity

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3.5  Ectoparasites and endoparasites

The 287 species of multicellular parasites in Table 1 all live inside the human body. They are the major category of multicellular parasite and are known as endoparasites (endon is Greek for ‘within’). Endoparasites can invade vital organs or live in the gut, bloodstream or tissues. The endoparasites of humans belong to four types of worms:

  • roundworms
  • tapeworms
  • filarial (thread) worms
  • flukes (or flatworms).

The examples that follow illustrate the often forgotten impact of multicellular endoparasites on human health and the variation in their life cycles and transmission routes. However, before we go on, it is worth observing that there exists an additional category of multicellular parasites: ectoparasites.

Ectoparasites are invertebrates (animals without backbones) that live on the surface (ektos is Greek for ‘outside’) of the human body, such as head lice, body lice and ticks. Their bites can cause intense irritation and they also transmit some potentially life-threatening pathogens to humans, such as the bacteria that cause typhus [tye-fuss]. People with typhus develop a very high fever (body temperature well above the normal 37 °C), severe headaches, muscle pain and a dark rash. It can spread rapidly in overcrowded communities, where lice and ticks proliferate because people cannot easily wash themselves or their clothes (Figure 4).

Figure 4  Infectious diseases are easily transmitted in the overcrowded conditions of refugee camps, like this one on the Burma (Myanmar)–Thailand border.