Some diseases jump from person to person; others never do. The difference is a pathogen, a living thing that causes disease and can be carried from one host to the next. Follow an outbreak through a small town, switch on the five public-health controls, then see the body's own barriers at work. Cholera is Extended content.
A pathogen is a disease-causing organism. A transmissible disease is one in which the pathogen can be passed from one host to another, either by direct contact (including blood and other body fluids) or indirectly (from contaminated surfaces, food, animals and air). The body keeps most pathogens out with skin, nose hairs, mucus and stomach acid, and white blood cells deal with those that get in. Clean water, hygienic food preparation, personal hygiene, waste disposal and sewage treatment break the routes of transmission.
Press Start an outbreak. One person (red) is ill. Grey people can still catch the disease and green people have recovered. Watch the number caught so far climb as the pathogen uses every route the town gives it: people bumping into each other, green marks left on surfaces, the well, the food stall and flies from the rubbish heap.
A pathogen is the organism that causes the disease, not the disease itself. Cholera is the disease; the cholera bacterium is the pathogen.
Pick each defence in turn. Most pathogens never get inside at all; the first four defences are barriers and traps. White blood cells are the last line, for the pathogens that do get through.
| Defence | How it works |
|---|---|
| Skin | a tough barrier pathogens cannot cross unless it is cut; if it is cut, blood clots to seal the wound |
| Hairs in the nose | trap dust and pathogens in the air breathed in |
| Mucus | made by cells lining the airways; traps pathogens, and cilia sweep it up to the throat to be swallowed |
| Stomach acid | hydrochloric acid kills most pathogens swallowed in food and drink |
| White blood cells | phagocytes engulf and digest pathogens; lymphocytes make antibodies |
Stomach acid kills pathogens; it does not "digest" them like food. And skin is a barrier only while it is unbroken, which is why a cut must clot quickly.
Extended. Press Drink contaminated water. The green rods are cholera bacteria in the small intestine. Watch the purple toxin, the yellow chloride ions and the blue water, and keep an eye on the blood volume and ions in blood readouts. Then try oral rehydration solution.
The toxin does not pump water directly. It moves chloride ions into the gut, and water follows by osmosis. Mention both steps in an explanation.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the outbreak sim, what colour shows a person who has recovered from the disease?
In the town, which control stops the flies from carrying the pathogen?
In the defences tab, what moves the mucus with its trapped pathogens up to the throat?
Extended: in the cholera tab, what happens to the readout for blood volume after the contaminated water is drunk?
Back in the outbreak, switch on the controls one at a time and run the outbreak again. Each control shuts one indirect route. With all five in place only direct contact remains, so far fewer people catch the disease.
| Control | Route it breaks |
|---|---|
| Clean water supply | pathogens in drinking water, such as cholera from a well polluted by sewage |
| Hygienic food preparation | pathogens passed on in food: wash hands and surfaces, cook food fully, keep raw and cooked food apart |
| Good personal hygiene | hand washing after the toilet and before eating removes pathogens picked up from surfaces |
| Waste disposal | rubbish attracts flies and rats that carry pathogens to food |
| Sewage treatment | pathogens in human faeces are removed before water returns to rivers and wells |
Hygiene measures stop pathogens reaching people. They do not make anyone immune, so they must be kept up all the time.
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Which statement describes a transmissible disease?
A nurse catches hepatitis B after a needle-stick injury. How was the pathogen transmitted?
Which defence kills pathogens that are swallowed in food?
Extended: why does a person with cholera lose water from the blood?
This skill is now recorded on your profile. Keep the chain going.