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Diseases and Immunity: Question 10

Syllabus 10.1

Multiple choice Core 1 mark

In a region near a large area of stagnant water, many villagers become ill with malaria, a disease caused by a single-celled organism that lives inside red blood cells. Investigators find that villagers who sleep under mosquito nets are far less likely to become ill than those who do not, even though none of the villagers who fell ill had any direct contact with each other.

Which statement correctly identifies the type of pathogen that causes malaria, and correctly explains how it is transmitted between villagers?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Identify the type of pathogen

Pathogens are not all the same kind of organism; they include bacteria, viruses, fungi and protoctists (single-celled organisms with a nucleus). The malaria pathogen is described as a single-celled organism that lives inside red blood cells. This matches a protoctist, not a bacterium (which does not have a nucleus) and not a virus (which is not a living cell at all).

Step 2: Work out how the pathogen is transmitted

None of the villagers who became ill had direct contact with each other, so the pathogen cannot have spread by direct transmission between them. Instead, the key clue is that villagers who slept under mosquito nets were far less likely to become ill. This points to a vector, an organism that carries a pathogen from one host to another, in this case a mosquito that bites an infected person, picks up the pathogen, and then transmits it to the next person it bites.

Because the pathogen travels between hosts by way of another organism (the mosquito), rather than through direct contact, this is a form of indirect transmission.

Step 3: Rule out the other options

  • A: malaria’s pathogen is a protoctist, not a bacterium, and drinking or sharing water is not the route suggested by the mosquito-net evidence.
  • C: the malaria pathogen is a protoctist, not a virus, and “damp air” is not a recognised transmission route here; the mosquito-net evidence points specifically to mosquitoes.
  • D: coughing spreads pathogens through the air by direct or indirect droplet transmission, but this does not fit a disease controlled by mosquito nets, and in any case transmission by a vector such as a mosquito is indirect, not direct.

Final answer

  • The malaria pathogen is a protoctist, transmitted indirectly between villagers by mosquitoes acting as a vector, option B.