1. 1
   An outbreak of a diarrhoeal illness occurred in a village after its water supply became contaminated with a disease-causing bacterium. Health officials traced the source to untreated sewage entering the village well and began investigating how the disease had spread between villagers.

   (a)

   What term describes an organism, such as this bacterium, that causes disease?

   [1 mark]

   • APathogen
   • BAntigen
   • CAntibody
   • DAntibiotic

   (b)

   By which method was the disease most likely transmitted between villagers in this outbreak?

   [1 mark]

   • ADirectly, through sexual contact only
   • BIndirectly, through contaminated water
   • CIndirectly, through airborne droplets only
   • DDirectly, through inherited genes

   (c)

   Suggest two measures the health officials could recommend to control the spread of this disease in the village.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Laboratory tests confirmed that the village outbreak was caused by cholera bacteria. Doctors explained to patients that the bacteria remained in the gut and released a toxin, rather than spreading through the bloodstream, and that severe dehydration was the main danger.

   (a)

   Laboratory tests confirmed that the outbreak in the village was caused by cholera bacteria. Which of the following best describes how cholera bacteria cause the severe diarrhoea seen in affected patients?

   [1 mark]

   • AThe bacteria block the bile duct, preventing fat digestion
   • BThe bacteria directly destroy the lining of the small intestine, preventing any water absorption
   • CThe bacteria produce a toxin causing secretion of chloride ions into the small intestine, leading to osmotic movement of water into the gut
   • DThe bacteria produce a toxin that increases water absorption from the gut into the blood

   (b)

   Why is severe dehydration the main danger for patients suffering from cholera?

   [1 mark]

   • AThe cholera bacteria consume the patient's blood glucose directly
   • BThe cholera toxin destroys red blood cells, reducing oxygen transport
   • CThe cholera toxin prevents the kidneys from producing urine at all
   • DLarge amounts of water and ions are lost from the body in watery diarrhoea

   (c)

   Explain, in terms of osmosis, why cholera toxin causes large amounts of water to move into the gut, resulting in watery diarrhoea.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Several years after recovering from a natural cholera infection, one villager remained protected from becoming ill again after later exposure to the same bacteria, while a visitor to the village who had never previously been exposed became severely ill after drinking contaminated water.

   (a)

   Explain, in terms of active immunity, why the villager was protected.

   [3 marks]

   (b)

   A vaccination campaign was later introduced in the village to protect people, including babies too young to have been naturally infected, against cholera. Explain how vaccination works to protect against a disease, and explain how a breastfeeding baby might gain some short-term protection even before being vaccinated.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A public health team ran an education programme in a region prone to cholera outbreaks, explaining to residents both the body's natural defences against pathogens and the importance of a planned vaccination programme in preventing future outbreaks.

   (a)

   Describe the body's main defences against pathogens entering the body, and explain how active immunity, gained either through infection or vaccination, provides longer-term protection than these initial defences alone.

   [6 marks]

   (b)

   The public health team found that some elderly residents, who had been vaccinated against cholera many years earlier but never naturally infected since, still became ill during a new outbreak, while younger adults vaccinated at the same time remained protected. Suggest and discuss possible biological explanations for this difference in protection between the two groups.

   [6 marks]

   Total for question 4: 12 marks

End of questions