Cambridge O Level Biology · Syllabus 5090 · Disease and Immunity
Cholera
What is Cholera?
A transmissible disease caused by a bacterium, transmitted mainly in drinking water contaminated with faecal material from an infected person. The bacterium produces a toxin in the small intestine that causes severe watery diarrhoea, and the resulting loss of water and ions from the blood is what makes the disease dangerous.
This definition is part of the Disease and Immunity chapter in Cambridge O Level Biology.
Common mistakes with Cholera
- "Cholera diarrhoea happens because water is not absorbed." That describes a different and far milder situation. In cholera, water is actively drawn out of the blood and tissues into the gut. Write The toxin causes chloride ions to be secreted into the lumen; the water potential of the lumen falls; water moves in from the blood and tissues by osmosis.
- "Cholera bacteria make holes in the intestine wall." The wall stays intact. What changes is what it transports. Write The bacteria produce a toxin that changes the movement of chloride ions across the intestinal cells.
Questions students ask about Cholera
How much detail about sewage treatment do I need?
None of the stages. The syllabus asks you to know that safe disposal of faeces and sewage treatment prevent faecal material contaminating water, and that this breaks the faecal–oral route by which cholera is transmitted. Describing settling tanks and filters is not required and earns nothing.