IGCSE Biology 0610: Pathogens, Transmission and Body Defences

St. Aldric's boarding school has a single drinking-water dispenser in one of its dormitories. During one term, staff failed to clean the dispenser's tap for several weeks, and a thin film of grime built up around the spout. In the same week, twenty-three students who drank from this dispenser developed vomiting and diarrhoea. Students in other dormitories, who used a separate, regularly-cleaned dispenser, remained healthy. Laboratory tests showed that all of the ill students were infected with the same species of bacterium.

(a) State what is meant by: (i) a pathogen (ii) a transmissible disease. [2]

(b) State whether the bacterium was transmitted to the students directly or indirectly, and explain your answer using information from the scenario. [2]

(c) Once swallowed in the contaminated water, the bacteria still had to overcome some of the body's defences before they could cause illness in a student. State two of the body's defences against pathogens that these bacteria would have had to overcome, and explain how each defence normally protects the body. [4]

Part (a): Defining pathogen and transmissible disease

(i) Pathogen: a disease-causing organism.

(ii) Transmissible disease: a disease in which the pathogen can be passed from one host (an infected person) to another host (a person who was not yet infected).

Part (b): Direct or indirect transmission?

The bacterium was transmitted indirectly.

None of the ill students caught the illness from direct contact with each other or with an infected person’s body fluids. Instead, every one of them drank from the same dispenser, whose tap had not been cleaned and had built up a film of grime. The bacterium was picked up from this contaminated surface each time a student filled a cup, and then swallowed in the water itself. Because the bacterium travelled by way of a contaminated object (the tap) rather than by contact between the students, this is indirect transmission.

Part (c): Body defences against the swallowed bacteria

Two of the body’s defences that these swallowed bacteria would have had to get past are:

Stomach acid: the stomach’s contents are strongly acidic. This low pH kills many of the bacteria that are swallowed in contaminated food or drink before they can reach and colonise the intestine.
White blood cells: any bacteria that survive the stomach’s acid and enter the tissues of the body can still be attacked by white blood cells, which destroy pathogens, for example by engulfing them in a process called phagocytosis.

Skin and the hairs in the nose are not relevant here, since these defend against pathogens entering through cuts in the skin or through the airways, not against pathogens that have been swallowed.

Final answers

(a) (i) A pathogen is a disease-causing organism. (ii) A transmissible disease is a disease in which the pathogen can be passed from one host to another.
(b) Indirect transmission, via the contaminated tap on the shared dispenser.
(c) Stomach acid (kills many swallowed pathogens with its low pH) and white blood cells (destroy pathogens that get past the stomach, e.g. by phagocytosis).