Disease and geography

Pathogens, transmission and the language of disease

A pathogen is a microorganism that causes disease, and the syllabus limits you to three kinds: a virus, a bacterium and a parasite. A communicable disease is one that can be passed from one person to another, while a non-communicable disease such as type 2 diabetes, lung cancer or a stroke cannot. The distinction is geographical, not just biological: communicable diseases move through populations and along routes, so their distribution changes in weeks and responds to water, housing, transport and healthcare, while non-communicable diseases accumulate slowly and reflect diet, behaviour, occupation and the age structure of the population.

The syllabus draws a fine distinction that is easy to lose marks on. Contagious means caught from other people; infectious means caught from the environment. Measles is contagious, because it passes from a cough to a nearby person. Cholera is infectious, because the bacterium is picked up from contaminated water or food and not from the breath or the touch of a patient. Influenza is both - contagious through droplets and infectious through contaminated hands and surfaces.

The route by which a pathogen leaves one host and enters the next decides almost everything else: who is at risk, which places it can reach, and what will stop it. There are six routes to know, and each one points to a different control. Airborne and droplet transmission is defeated by ventilation, masks and distance; waterborne transmission by clean water and sanitation; vector transmission by nets, spraying and drainage.

Prevalence and the four words for how far a disease has spread

Prevalence is the proportion of a population that has a disease at a given time. It is usually expressed as a rate per 1 000 or per 100 000 people, or as a percentage, so it allows you to compare a small city with a large country. High prevalence means a large share of the population is ill now, which matters because that is the share that cannot work, cannot go to school and needs treatment.

Four words describe how far and how fast a disease is spreading, and examiners expect the scale to be right. An outbreak is a rise in cases above the level expected in a small, defined area, such as one town or one camp. An epidemic is a rise above the expected level across a country or a region. A pandemic is an epidemic that has spread across a world region or globally, so that several continents are affected at once. Endemic is different in kind rather than in degree: a disease is endemic when it is permanently present at a predictable, expected level in a population, which is why malaria can be described as endemic in much of sub-Saharan Africa even in a year when cases are not rising.

Two traps follow from those definitions. First, pandemic describes geographical spread and not how dangerous a disease is - the 2009 H1N1 influenza pandemic was caused by a strain that turned out to be milder than feared for most people, but it reached more than 200 countries. Second, endemic does not mean mild and epidemic does not mean severe: the same disease can sit at a stable endemic level for years and then flare into an epidemic when immunity falls, water supplies fail or people are displaced.

Measuring the impact of disease: social and economic measures

The syllabus gives four social measures. Morbidity is the number or proportion of people living with a disease, so it measures illness rather than death. The death rate is the number of deaths per 1 000 people per year. The infant mortality rate (IMR) is the number of deaths of children under one year old per 1 000 live births per year. Life expectancy is the average number of years a baby born in a particular year would live if current death rates stayed the same.

Together they describe both the burden of illness and its lethal outcome, which is why a country can have high morbidity from malaria and still show a falling death rate if treatment improves.

Age/sex structure diagrams (population pyramids) turn those numbers into a picture and link disease directly to population structure. A low-income country typically has a broad base and a narrow top: many children, a high birth rate, a high IMR and few elderly people, so the disease burden is dominated by infections of childhood, diarrhoeal disease and maternal conditions. A high-income country has a narrow base and a wide top: fewer births, low infant mortality and a large elderly population, so the burden shifts to cancers, heart disease, stroke and dementia. A country in the middle of that transition often carries both at once.

The economic measures work differently. GDP is the total value of goods and services a country produces in a year, and GDP per head is the better measure for comparing health because it shows what is available per person. Spending on healthcare can be measured as a share of GDP, as money per person, or by who pays - government, insurance or the patient out of their own pocket, which is the figure that decides whether a poor family can afford treatment. Workforce issues are the human side: days lost to illness, the productivity of a weakened labour force, the cost of caring for the sick, the loss of working-age adults to disease, and the emigration of doctors and nurses from poorer to richer countries.

The global pattern of disease and the factors behind spatial variation

The World Health Organization (WHO) reports the main causes of death by country income group, and the pattern is the clearest evidence that disease is a development issue. In low-income countries (LICs) the leading causes are communicable diseases and conditions of pregnancy and early childhood: lower respiratory infections, diarrhoeal disease, malaria, tuberculosis, HIV and neonatal conditions. In high-income countries (HICs) the leading causes are non-communicable: ischaemic heart disease, stroke, cancers of the lung and airways, dementias and chronic lung disease. Middle-income countries (MICs) sit between the two and often carry a double burden, with rising heart disease and diabetes alongside continuing tuberculosis, dengue and diarrhoeal disease. The same broad shift, from communicable to non-communicable causes as a country develops, is what geographers call the epidemiological transition.

Demographic and environmental factors explain much of the variation within countries as well as between them. Age structure decides who is vulnerable: measles and diarrhoea kill children, malaria kills children under five and pregnant women, and heart disease kills the elderly. Climate sets the limits of vector-borne disease, because the parasite or virus has to develop inside the insect, and that takes warmth; water quality and quantity decide whether a waterborne disease can spread and whether people can wash; and environmental conditions such as standing water, humidity, vegetation and drainage decide whether the vector can breed and survive. Sanitation and healthcare infrastructure turn a pathogen into an epidemic or contain it - sewers, chlorinated supplies, clinics within reach, staff, medicines and a laboratory that can confirm a diagnosis.

Disaster events cause sudden spatial variation by destroying exactly the infrastructure that controls disease. Flooding contaminates wells and leaves standing water for mosquitoes; tropical cyclones destroy water pipes, power, roads and health facilities across a wide area at once and force people into shelters; earthquakes break sewers and water mains and trap survivors in crowded, unsanitary camps. In each case the hazard does not create the pathogen - it redistributes people and destroys the defences, which is why cholera and other diarrhoeal diseases so often follow floods and earthquakes.

The remaining factors are human and they interact. Social: living conditions, crowding, diet, education and behaviour such as smoking or exercise. Economic: poverty or affluence shaping diet and lifestyle, occupation (mining, farming and sex work carry particular risks), the level of economic development, investment in healthcare, and the vulnerability of infrastructure that was built cheaply. Political: governance and corruption, whether the state invests in health, conflict that destroys services, and international relations that decide whether aid, vaccines and expertise arrive. Global interconnectedness: migration and travel, which move pathogens between places faster than any vector could, and which also move health workers away from the countries that trained them.

Influenza: a viral disease

Influenza is caused by a virus and spreads mainly by the airborne route: an infected person coughs or sneezes out droplets and aerosols, and someone nearby breathes them in. It also spreads by indirect contact, when hands pick the virus up from a contaminated surface and carry it to the mouth, nose or eyes. That is why the syllabus asks about factors influencing spread rather than about the biology: the virus travels where people are close together and indoors, so crowding in homes, schools, workplaces, public transport and care homes drives transmission, and cold, dry winter air and closed windows help it, which produces the sharp seasonal peaks of temperate countries. In the tropics the seasonality is weaker and outbreaks are less predictable.

Prevalence and risk factors are best expressed as an annual burden rather than a single figure: in a typical year, roughly 5 to 10% of adults and 20 to 30% of children are infected worldwide. Most recover in a week or two, but the people at risk of severe illness and death are the over-65s, pregnant women, young children, people with chronic heart or lung disease or diabetes, and people whose immune system is suppressed, together with health workers who are exposed repeatedly. Deaths are concentrated in those groups, which is why an influenza epidemic can raise the death rate sharply in an ageing HIC while barely changing it in a country with a young population.

The global distribution of influenza is unusual: it is present everywhere, but not everywhere at once. It circulates continuously in the tropics and spreads outwards each year along travel and transport routes, so the pattern is better described as waves moving through space and time than as a map of countries with and without the disease. Air travel compresses that: in 2009 the new H1N1 strain was identified in Mexico and the United States in April and had reached more than 200 countries within weeks, and the WHO declared a pandemic on 11 June 2009. The social and economic impacts follow from the same feature - it removes a large share of the workforce at once. Schools close, hospitals are overwhelmed, businesses lose productivity and tourism falls, and the costs fall on health services and on households at the same time.

The reason influenza never goes away is that the virus changes. Small, gradual changes (antigenic drift) alter the surface proteins enough that immunity from previous infections and vaccines no longer matches, which is why the vaccine is reformulated every year and why you can catch influenza repeatedly. A major change (antigenic shift), usually when a strain from an animal population acquires the ability to infect people efficiently, produces a virus that almost nobody has immunity to - the condition for a pandemic. Pigs can be infected by both bird and human influenza viruses and can act as a mixing vessel in which strains reassort, and birds, especially poultry, are the source of strains such as H5N1 and H7N9 that have infected people in close contact with them. The risk is not that an animal strain is always severe, but that a new human strain can spread through a population with no immunity before a vaccine exists.

One country's strategy: the United Kingdom. The UK plans for influenza in advance and treats a new strain as a national emergency. It holds a stockpile of antiviral medicines so that treatment can begin before a vaccine is ready, and it maintains virological and clinical surveillance - samples from a network of general practices and hospitals - so that a new strain is detected and its severity assessed early. In the 2009 pandemic it set up the National Pandemic Flu Service, a telephone and internet assessment service that authorised antiviral collection without a visit to a doctor, which kept infectious patients out of surgeries and hospitals and protected the staff who had to keep treating everyone else. It ran a vaccination programme for at-risk groups once a vaccine was available, promoted simple hygiene messaging (catch it, bin it, kill it), used school closures where local outbreaks were severe, and coordinated the response between the four nations' health services. The lessons are worth stating: speed matters more than perfection, antivirals and vaccines only help if they reach the people at risk, and a health system that is already short of beds has no spare capacity when a large share of the population falls ill at once.

Cholera: a bacterial disease

Cholera is caused by the bacterium Vibrio cholerae and is spread by the waterborne route, with food as a secondary route. The bacterium leaves an infected person in the faeces and enters the next person in contaminated drinking water, in food washed or irrigated with that water, or in shellfish from contaminated water. It is not contagious in the everyday sense: you catch it from the environment, not from the breath of a patient. The illness is severe watery diarrhoea - the classic 'rice-water' stool - which causes rapid dehydration, and a patient can lose several litres of fluid a day, so death can follow within hours without treatment.

The factors that influence spread are almost all about water and sanitation. A supply that is unchlorinated, intermittent or piped through broken mains lets sewage in; open defecation, overflowing latrines and blocked drains put the bacterium where people wash and drink; crowding in camps and informal settlements concentrates the risk; and floods spread contaminated water over wells and fields. Warm temperatures help the bacterium survive and multiply in water, and low levels of immunity in a population that has never been exposed means an outbreak can take hold quickly. Poverty and conflict are the underlying factors, because both prevent the investment that would fix the water supply.

Prevalence and risk factors. Cholera is endemic in parts of Africa and South Asia, where cases appear every year in the same places and children under five are most often affected. Elsewhere it arrives as an epidemic in a population with no immunity, and then all ages are vulnerable. With prompt treatment the case fatality rate can be kept below 1%; if patients cannot reach treatment it can rise well above 5%, which is why the same disease kills a few hundred people in one country and tens of thousands in another. The global distribution is therefore a map of water and sanitation failure rather than of climate: the seventh pandemic, which began in 1961, has left cholera established across much of Africa and South Asia, and the largest outbreaks of recent decades have been in Haiti from 2010 and Yemen from 2016.

The risk after disaster events is the part the syllabus singles out. An earthquake, flood or cyclone destroys the water and sewerage network, drives people into camps where they share latrines and water points, and interrupts the health services and laboratories that would normally detect and treat cases. Haiti shows the sequence: the earthquake of January 2010 destroyed water and sanitation in the capital, people moved into camps, and cholera arrived in October 2010 with a peacekeeping contingent from Nepal and spread rapidly along the Artibonite river. Cyclone Idai in Mozambique in 2019 produced the same pattern of flooding followed by cholera. The pathogen has to be introduced, but the disaster creates the conditions in which it explodes.

Social and economic impacts are severe for a disease that is cheap to treat. Families lose income while they are ill or caring for the sick, children miss school, and the fear of the disease stops people buying food from markets and traders, so the economic damage reaches beyond the patients. Health services are overwhelmed at exactly the moment their buildings, staff and supplies have been damaged, and governments must spend on emergency water trucking, chlorine and treatment centres instead of on development. In the longer term, repeated outbreaks keep investors and tourists away and hold back the water and sanitation investment that would end them.

One country's strategy: Haiti after 2010. The response used the fact that cholera is simple to treat and simple to prevent. Oral rehydration was pushed down to the community: rehydration points were set up in the affected neighbourhoods, and oral rehydration salts and training were given to families and community health workers so that treatment could start before a patient reached a clinic. Cholera treatment centres were established to isolate and rehydrate severe cases, water supplies were chlorinated and trucked to camps, and hygiene promotion explained handwashing, safe storage and the use of latrines. From 2013 oral cholera vaccine campaigns were added in the worst-affected areas, and longer-term work concentrated on piped water, sewers and waste management. The honest evaluation is that treatment saved many lives and the case fatality rate fell, but the outbreak was not brought under control until the water and sanitation infrastructure improved, because vaccination and treatment manage a waterborne disease while only clean water and sewers stop it.

Malaria: a parasitic disease spread by a vector

Malaria is caused by a parasite of the genus Plasmodium - P. falciparum causes most severe illness and deaths, and P. vivax is widespread in Asia and Latin America - and it is transmitted by a vector, the female Anopheles mosquito. When the mosquito bites an infected person it takes up the parasite, which develops inside the insect for a period of days before it can be passed on in the next bite. That development needs warmth, which is why temperature sets the geographical limits of the disease.

Mosquitoes bite mainly between dusk and dawn, breed in standing or slow-moving water, and rest in shade and vegetation, and each of those habits is a point of control.

The environmental requirements explain the map. The parasite cannot complete its development in the mosquito below about 16-18 °C, so malaria is absent from highlands above the temperature limit and from cool seasons; it needs standing water for breeding, so it peaks in and after the rainy season and is scarce in deserts and in well-drained, sewered cities; and it needs humidity and vegetation for the adult mosquito to survive. Altitude, drainage, rainfall and land use therefore produce sharp differences in prevalence within a single country, and irrigation schemes, dams, rice paddies and deforestation can all increase transmission by creating breeding sites.

Prevalence, distribution and risk factors. Malaria is concentrated in the tropics and subtropics, and the WHO African Region accounts for about 95% of cases; Nigeria, the Democratic Republic of the Congo, Uganda and Mozambique together account for around half of the world total. The people most at risk are children under five, who have not yet built up immunity and in whom the disease can kill within days, and pregnant women, for whom infection causes anaemia and low birth weight. Adults in high-transmission areas acquire partial immunity through repeated infection, which is why the same parasite kills children and merely weakens adults, and why that immunity is lost by people who move away and return.

Spread due to changing environmental conditions is the syllabus's forward-looking point. Warming temperatures extend the transmission season and allow the mosquito and the parasite to survive at higher altitudes and higher latitudes, which puts populations with no immunity at risk in the highlands of East Africa and in the subtropics. Changes in rainfall matter as much as temperature: more intense rain fills breeding sites, while drought can make people store water in containers near their homes, which does the same. Urbanisation with drainage reduces transmission, while irrigation, mining pools and construction sites increase it, and resistance to antimalarial drugs and to the insecticides used on nets and walls has become a serious constraint.

The social and economic impacts run through households and schools. Malaria causes repeated episodes of fever and anaemia, so children miss school and adults lose days of farm work at exactly the season when labour is needed; families spend money on treatment and transport and lose income while caring for the sick; and the disease contributes to anaemia in pregnancy and to low birth weight. At national scale, repeated illness lowers productivity and deters investment, and the cost of control programmes is a permanent charge on health budgets - the poor country pays twice, once in illness and once in prevention.

One country's strategy: Sri Lanka. Sri Lanka eliminated malaria and was certified malaria-free by the WHO in September 2016, after decades in which the disease had been endemic across much of the island. The programme worked on several fronts at once: indoor residual spraying of homes in endemic districts, insecticide-treated nets for people in high-risk areas, and case-based surveillance in which every confirmed case was investigated, its location mapped and its contacts and surroundings checked for breeding sites. Mobile malaria clinics reached remote and conflict-affected areas where people would not travel to a hospital, entomological surveillance monitored mosquito numbers and insecticide resistance, and active case detection screened high-risk groups such as soldiers, migrant workers and people returning from abroad. Treatment was free, which removed the cost barrier that keeps poor patients away, and the programme was maintained after cases fell to zero, because the last few cases are the ones that decide whether elimination holds. The lesson is that elimination is a surveillance achievement as much as a medical one.

Monitoring disease and controlling it

Monitoring means collecting, testing and reporting data on who is ill, where and with what, so that a response can be aimed at the right place at the right time. The syllabus lists the factors that decide whether monitoring works, and they are as much political and economic as scientific. Cost-effective testing matters because a test that costs more than the treatment will not be used at scale in a poor country, which is why rapid diagnostic tests that need no laboratory have changed what is possible. Access and scalability of testing technology decides whether a country can go from a few hundred tests a week in a national laboratory to tens of thousands across rural clinics. Access to patients decides whether cases are found at all: people who live far from a clinic, who cannot take a day off work, or who fear stigma are never tested, so the map of reported cases becomes a map of testing rather than a map of disease.

Data accuracy is the next constraint. Cases are under-reported where diagnosis depends on a laboratory that is far away, where deaths are not registered, and where a government has a reason to play down an outbreak. Disclosure and patient confidentiality have to be balanced against each other: contact tracing needs named individuals and their movements, but people will avoid testing if they expect their identity, their HIV status or their occupation to be revealed. International relations decide whether a country shares genetic sequence data and case numbers quickly, whether it receives reagents, tests and vaccines, and whether its citizens are believed - a country that is punished with travel bans for reporting a new variant has an incentive to report late next time.

Controls work at different points in the chain of transmission. Vaccines protect the individual before exposure and, at high coverage, protect the population by reducing transmission. Education changes the behaviour that puts people at risk - handwashing, safe water storage, sleeping under a net, wearing a mask, seeking treatment early. Sanitation infrastructure - piped and chlorinated water, sewers, latrines, drainage and waste collection - removes the route by which waterborne disease travels, and is the only control that ends a cholera epidemic permanently. Low-tech solutions are the ones that work where money and electricity are short: mosquito nets, face masks, oral rehydration salts, chlorine tablets, soap, and simple barriers between patients and vectors.

The choice of control has to match the route: a vaccine and a mask for an airborne virus, water and sewers for a waterborne bacterium, a net and a sprayed wall for a vector-borne parasite.

Responding to outbreaks: the factors that decide success, and two contrasting countries

An effective response needs four things at once, and the syllabus asks for them by category. Environmental: whether the pathogen can be contained by the physical setting - an island or a remote border is easier to protect than a long land frontier, and climate affects how long it survives outside a host and whether vectors are active. Social: whether people understand the advice, trust the government and can act on it - which depends on education, crowded housing, the need to earn a daily wage, and the willingness to isolate. Economic: whether the state can pay for tests, hospitals, oxygen, vaccines and wage support, and whether households can afford to stop working. Political: whether there is clear leadership, a coordinated plan, functioning local government, public confidence and useful international relationships.

Where all four are favourable, a response can suppress an outbreak; where any one fails, the others are wasted.

Two contrasting responses to one pandemic: New Zealand and South Africa, COVID-19 from 2020. New Zealand combined a favourable environment with strong economic and political capacity. It is an island with few entry points, so it closed its borders early and required quarantine at the border; it then used a national Alert Level system to impose a strict nationwide lockdown in March 2020, aimed at elimination rather than suppression, backed by widespread testing, intensive contact tracing and generous wage subsidies so that people could stay at home. Case numbers stayed very low for long periods and deaths were few, but the strategy depended on continued border restrictions, and the arrival of more transmissible variants showed that geography delays a pathogen rather than excluding it.

South Africa responded early and decisively in one sense - it declared a national state of disaster and imposed one of the strictest lockdowns in the world in March 2020, before many richer countries acted - but its capacity to sustain that response was much more limited. Social conditions worked against it: crowded housing in townships, shared water and toilet facilities and a large population depending on daily wages made isolation and handwashing impossible for many households, and a heavy burden of HIV and tuberculosis increased vulnerability. Economic constraints limited testing, oxygen, hospital beds and the wage support that could keep people at home, and political factors included contested restrictions, procurement difficulties and a vaccine rollout that began later and more slowly than in rich countries. Deaths per head were far higher than in New Zealand, and the difference was not the virus or the government's intent but the environment, the money and the living conditions.

The comparison is the exam answer. Both countries acted quickly and both had competent health authorities; the outcomes diverged because one could close its borders, fund its households and isolate its cases, and the other had to do the same things in dense, unequal settlements with a health system already carrying HIV and TB. That is the syllabus's point in one sentence: a response is effective when environmental, social, economic and political factors all support it, and the weakest factor sets the limit.