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.