Each disease can be attacked at three points: the infected person, the route and the next host. Whether a measure works depends on the biology of the pathogen, on how people live and behave, and on what a country can afford.
Control measures break the chain of transmission at one of three points: treating infected people so they stop passing the pathogen on, blocking the route (clean water, mosquito nets, condoms) or protecting the next host (vaccination). Each one is limited by biological factors (drug and insecticide resistance, pathogens hiding inside cells, many strains or changing antigens, symptomless carriers, no vaccine for HIV), social factors (crowding in refugee camps and cities, natural disasters and war, stigma, whether people use nets or condoms or finish treatment) and economic factors (the cost of sewage systems, drugs, nets and long courses of treatment, often in the poorest countries). Details of the malaria life cycle are not needed.
Choose a disease, leave every box unticked and press Run the outbreak: one infected person enters a community of 200. Then tick measures one at a time and run it again. Watch the red line (ill now) and the yellow line (total ever infected).
| Disease | Measures in the model | What each one does |
|---|---|---|
| cholera | clean water supply, sewage treatment, oral vaccine, hygiene education, oral rehydration | the first four cut transmission; oral rehydration solution replaces lost water and ions, saving lives, but does not stop spread |
| malaria | insecticide-treated bed nets, spraying insecticide, draining standing water, antimalarial drugs, vaccine (partial) | reduce bites or mosquito numbers; drugs shorten the time a person is infectious |
| TB | BCG vaccination; testing, contact tracing and DOTS treatment; better housing; pasteurised milk and testing cattle | protect the susceptible, cure cases sooner, reduce crowding, remove the cattle source |
| HIV | condoms; testing and antiretroviral treatment; clean needles and screened blood; education | block the route; antiretrovirals lower the amount of virus so a person is far less infectious |
The model is illustrative, not real data, but it shows the key idea: once each infected person passes the pathogen on to fewer than one new person on average, the outbreak dies out.
A treatment that saves lives does not always reduce transmission. Oral rehydration solution prevents death from dehydration in cholera, but the patient still releases bacteria in faeces. In the model the curves do not change when only that box is ticked.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the prevention and control tab, what does the yellow line on the graph show?
For cholera, which measure leaves the outbreak curves unchanged when it is the only box ticked?
Which measure does the lab offer for TB but not for any other disease?
The explanation panel for each disease sorts the factors into three groups. What are they?
Exam questions ask you to discuss the factors. Sort each point into the right column and link it to the disease.
| Disease | Biological factors | Social factors | Economic factors |
|---|---|---|---|
| cholera | many strains; the oral vaccine gives only partial, short-lived protection; symptomless carriers spread bacteria unknowingly | refugee camps, slums and natural disasters destroy sanitation; hand washing and safe food handling need education | piped clean water and sewage treatment are expensive to build; oral rehydration solution is cheap |
| malaria | Plasmodium is resistant to many drugs; Anopheles becomes resistant to insecticides; the parasite lives inside liver and red blood cells and changes its antigens, so vaccines are only partly effective; mosquitoes breed in any small pool | nets must be used every night; people in remote areas or war zones cannot reach clinics; spraying and drainage need community cooperation | drugs, nets and spraying programmes cost money in some of the poorest countries; illness keeps people from work and school |
| TB | treatment needs several antibiotics for at least six months; multiple drug-resistant TB is spreading; BCG protection varies; bacteria survive inside phagocytes; HIV makes people more vulnerable; cattle are a reservoir of M. bovis | overcrowding and homelessness; patients stop treatment when they feel better, so DOTS (taking each dose under supervision) is used; stigma delays diagnosis | long courses of several drugs, testing, contact tracing and supervised treatment are costly; testing cattle and pasteurisation cost money |
| HIV | no vaccine and no cure: HIV mutates rapidly, changes its antigens and hides inside host cells; antiretrovirals control the virus but must be taken for life | stigma stops people being tested; cultural and religious attitudes to condoms; people may not know they are infected for years | lifelong antiretrovirals and testing are expensive; deaths of working-age adults harm families and national economies |
Do not just list measures. A discuss question needs the factor and its effect: for example, mosquitoes have become resistant to insecticides so spraying kills fewer of them and the vector survives to transmit Plasmodium.
Smallpox was eradicated by vaccination. The four diseases in this topic have features that make that much harder.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which is a biological reason why there is no vaccine against HIV?
Directly observed treatment (DOTS) for TB involves a health worker watching each dose being taken. What is the main reason for this?
Which is a social factor that makes cholera hard to control in a refugee camp?
Insecticide-treated bed nets reduce malaria cases most when they are used every night. Why are they effective?
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