Pathogenic diseases, their spread and impacts

14.2.1 begins with the MECHANISMS OF TRANSMISSION — the precise route a pathogen takes from one host to the next. Examiners expect you to match a NAMED disease to its route, because the route shapes how, where and how fast it spreads.

Vector-borne transmission. The pathogen is carried from host to host by another organism — a vector, usually a blood-feeding insect. The classic example is malaria, caused by the Plasmodium protozoan and carried by the female Anopheles mosquito; dengue (Aedes mosquito), sleeping sickness (tsetse fly) and schistosomiasis (water snails) are also vector-borne. Because the vector has habitat needs, vector-borne disease is tied to warm, wet, low-altitude regions.

Water-borne / faecal–oral transmission. The pathogen is swallowed in water or food contaminated by infected faeces. Cholera, caused by the bacterium Vibrio cholerae, is the textbook case: sewage contaminates drinking water, people drink it, and the disease spreads explosively where sanitation is poor. Typhoid and most diarrhoeal diseases travel the same faecal–oral route.

Airborne / droplet transmission. The pathogen is breathed out and inhaled in tiny droplets when an infected person coughs, sneezes or talks. Tuberculosis (TB), influenza, measles and COVID-19 all spread this way. Airborne disease spreads fastest where people are CROWDED and indoors.

Direct contact. The pathogen passes through touching an infected person or surface — including skin-to-skin contact (e.g. some fungal and skin infections) and contact with sores.

Bodily fluids. The pathogen passes in blood, semen or other fluids. HIV (the virus that causes AIDS) spreads through unprotected sexual contact, infected blood (shared needles, transfusions) and from mother to child during birth or breastfeeding. This route needs intimate or blood contact, so HIV spreads more slowly than airborne disease but is very hard to eliminate.

Transmission route	How it works	Named disease	Where it spreads fastest
Vector-borne	Carried by an organism (insect)	Malaria (Anopheles mosquito)	Warm, wet tropics
Water-borne / faecal–oral	Swallowed in contaminated water/food	Cholera (Vibrio cholerae)	Poor sanitation areas
Airborne / droplet	Inhaled in coughs/sneezes	TB, influenza, COVID-19	Crowded, indoor settings
Direct contact	Touching infected person/surface	Skin infections	Close personal contact
Bodily fluids	Blood, sexual contact, mother-to-child	HIV/AIDS	Via intimate/blood contact

The take-home for 14.2.1: every infectious disease has a SPECIFIC transmission route, and naming it precisely (malaria = vector-borne; cholera = water-borne/faecal–oral; TB and COVID-19 = airborne/droplet; HIV = bodily fluids) is the foundation of every answer about spread.

14.2.1 is anchored by NAMED case studies, and the essential vector-borne example is MALARIA. You must know its cause, its transmission cycle and the conditions it needs.

Cause and vector. Malaria is caused by the Plasmodium protozoan parasite (several species; P. falciparum is the deadliest). It is transmitted from human to human only by the bite of an infected female Anopheles mosquito — the female needs a blood meal to develop her eggs, and in feeding she both PICKS UP and PASSES ON the parasite. Malaria is therefore vector-borne, NOT contagious.

The transmission cycle. The cycle alternates between human and mosquito:

• A female Anopheles mosquito bites an infected human and takes up Plasmodium parasites in the blood.
• The parasite develops inside the mosquito over several days (this is why warmth speeds transmission — the parasite matures faster in heat).
• The mosquito then bites a healthy human, injecting parasites in its saliva.
• The parasites multiply in the human's liver and red blood cells, causing fever, anaemia and, in severe cases (especially P. falciparum), death. That person becomes a new RESERVOIR from which the next mosquito can pick up the parasite.

The conditions malaria needs. The Anopheles mosquito requires warm temperatures (roughly 16–35°C) and standing water (pools, paddy fields, puddles, water tanks) to lay eggs and breed. This is why malaria is concentrated in the warm, wet TROPICS and sub-tropics — sub-Saharan Africa carries about 95% of the world's malaria deaths — and is absent from cold climates and from high altitudes (generally above ~2000 m).

Stage of the cycle	What happens	Why it matters
Mosquito bites infected human	Female Anopheles takes up Plasmodium in blood meal	Mosquito becomes infective
Parasite develops in mosquito	Plasmodium matures over days, faster in warmth	Warm climate speeds transmission
Mosquito bites healthy human	Parasite injected in mosquito saliva	New human infected
Parasite multiplies in human	Liver and red blood cells; fever, anaemia, death	Human becomes new reservoir
Breeding in standing water	Larvae develop in pools, paddies, tanks	Standing water + warmth set the range

The take-home for 14.2.1: malaria is a VECTOR-borne disease in which the female Anopheles mosquito shuttles the Plasmodium parasite between humans; the cycle needs warmth and standing water, which confines malaria to the tropics and makes it a model for explaining how a vector-borne disease spreads.

To contrast with malaria, 14.2.1 needs a DIRECTLY-TRANSMITTED disease — one that does not need a vector. Two strong named examples are HIV/AIDS (bodily fluids) and CHOLERA (water-borne/faecal–oral).

HIV/AIDS — transmission through bodily fluids. HIV (human immunodeficiency virus) attacks the immune system; untreated, it leads to AIDS. It spreads ONLY through certain bodily fluids:

• Sexual contact — unprotected sex is the main route, especially in sub-Saharan Africa, which holds about two-thirds of the world's people living with HIV.
• Infected blood — shared needles among drug users, and (historically) unscreened blood transfusions.
• Mother to child — during pregnancy, birth or breastfeeding. Because it needs intimate or blood contact, HIV spreads more SLOWLY than airborne disease, but it is very hard to eliminate, has a long latent period (people spread it before symptoms appear), and antiretroviral therapy (ART) now turns it into a long-term condition rather than a quick killer — so its PREVALENCE is high.

Cholera — water-borne / faecal–oral transmission. Cholera is caused by the bacterium Vibrio cholerae and spreads through the faecal–oral route: faeces from an infected person contaminate water or food, which others then consume. It causes severe, rapid diarrhoea and dehydration that can kill within hours if untreated. Cholera explodes where SANITATION is poor and clean water is lacking — refugee camps, slums and disaster zones (e.g. the 2010 Haiti outbreak after the earthquake, and recurring outbreaks in Yemen). Because it travels in water, cholera can spread very fast through a population sharing a contaminated supply.

Feature	HIV/AIDS	Cholera
Pathogen	Virus (HIV)	Bacterium (Vibrio cholerae)
Transmission	Bodily fluids (sexual, blood, mother-to-child)	Water-borne / faecal–oral
Speed of spread	Slow (needs intimate/blood contact)	Fast (shared water supply)
Key control	Poverty, behaviour, healthcare access	Sanitation and clean water
Where worst	Sub-Saharan Africa	Slums, refugee camps, disaster zones

The take-home for 14.2.1: a directly-transmitted disease needs NO vector — HIV travels in bodily fluids and depends on behaviour and healthcare, while cholera travels faecal–orally and depends on sanitation — and contrasting one of these with malaria lets you show that DIFFERENT transmission routes produce different geographies and demand different controls.

14.2.1 also requires the DIFFUSION of disease — the way it spreads across SPACE and through populations. Examiners credit the named types of diffusion and the role of globalisation.

Expansion (contagious) diffusion. The disease spreads OUTWARDS from a source as infected people pass it to those nearby — like ripples on a pond. Cases multiply and the affected area grows while the source stays infected. This is typical of CONTAGIOUS, airborne disease (measles, influenza, COVID-19 in its early local spread), where person-to-person contact drives outward growth. Contagious diffusion is the contact-driven form of expansion diffusion, strongly controlled by distance — nearby people are infected first.

Hierarchical diffusion. The disease jumps DOWN a settlement hierarchy — from large cities to smaller towns to villages — rather than spreading evenly outward, because large, well-connected places have the most contacts and transport links. It also spreads along TRANSPORT NETWORKS (roads, railways, flight routes), so a disease can appear in a distant big city before reaching a nearby village. Hierarchical diffusion explains why epidemics often hit major hubs first.

Relocation diffusion. The disease is CARRIED to a new area by MIGRANTS or TRAVELLERS who move and take the pathogen with them, leaving the source behind. Historic examples include diseases carried by colonists and traders; modern examples include HIV and COVID-19 carried between countries by air travellers. Relocation diffusion is how disease crosses oceans and continents.

Barriers and accelerators. Barriers slow diffusion — physical distance, oceans, mountains and deserts; unfavourable climate (which halts a vector); and human responses such as quarantine, border controls, vaccination and acquired immunity. Accelerators speed it — above all GLOBALISATION and air travel, which let a disease jump continents in HOURS: COVID-19 spread worldwide from Wuhan in early 2020 along international flight routes, a textbook case of relocation + hierarchical diffusion supercharged by global connectivity.

Type of diffusion	How it spreads	Example
Expansion / contagious	Outward from a source by person-to-person contact	Measles, COVID-19 (local spread)
Hierarchical	Down a settlement hierarchy / along transport networks	Epidemics hitting big cities first
Relocation	Carried to a new area by migrants/travellers	HIV, COVID-19 between countries
Barriers	Distance, oceans, climate, quarantine, immunity	Quarantine slowing an outbreak
Accelerator	Globalisation and air travel	COVID-19 worldwide in weeks (2020)

The take-home for 14.2.1: disease diffuses by EXPANSION/contagious spread (outward by contact), HIERARCHICAL spread (down the settlement hierarchy and along transport), and RELOCATION (carried by travellers) — and globalisation and air travel now accelerate spread so much that an outbreak can become a pandemic in weeks.

14.2.2 turns to the IMPACTS of pathogenic disease — grouped into DEMOGRAPHIC, SOCIAL and ECONOMIC, and unevenly felt between LICs and HICs. The clearest illustration is HIV/AIDS in southern Africa, but malaria and cholera show the same categories.

Demographic impacts. Disease raises MORTALITY and lowers LIFE EXPECTANCY. Where a disease kills selectively, it DISTORTS the AGE–SEX STRUCTURE of the population. HIV/AIDS, for example, kills mainly young to middle-aged ADULTS, hollowing out the working-age band of the population pyramid and leaving a 'missing generation' — in the worst-affected countries (e.g. Botswana, Eswatini) life expectancy fell sharply in the 1990s–2000s before treatment recovered it. Malaria, by contrast, kills mainly young CHILDREN, raising infant and child mortality.

Social impacts. Disease damages families and society. AIDS leaves millions of ORPHANS (an estimated 13+ million in sub-Saharan Africa), often raised by elderly grandparents or older siblings. Diseases carry STIGMA (HIV, leprosy), which deters testing and treatment and isolates sufferers. The loss of parents and carers strains EXTENDED FAMILIES, and the surge of patients OVERWHELMS HEALTH SYSTEMS, diverting scarce doctors, nurses and beds from other care. Children may be pulled out of school to care for the sick or to earn income.

Economic impacts. Disease is a brake on DEVELOPMENT. It removes WORKERS from the economy through illness and death, cutting PRODUCTIVITY and output, and falls hardest on the young, productive workforce. It imposes huge HEALTHCARE COSTS on households and governments. It deters INVESTMENT and TOURISM (firms and tourists avoid disease-hit regions). Malaria alone is estimated to cost sub-Saharan Africa billions of dollars a year in lost output and is widely seen as both a cause and a consequence of poverty — a disease–poverty cycle that holds development back.

LIC vs HIC. The same disease hits LICs far harder. LICs have weaker health systems, less money for treatment and prevention, more poverty and poorer sanitation, so disease causes more deaths and deeper economic damage; HICs can fund treatment, vaccination and clean water, so the same pathogen causes far fewer deaths (e.g. cholera is trivial where water is clean; malaria has been eliminated from HICs). Disease therefore both REFLECTS and DEEPENS global inequality.

Impact type	What it means	Named example
Demographic	↑ mortality, ↓ life expectancy, distorted age–sex structure	HIV/AIDS hollowing the working-age band in southern Africa
Social	Orphans, stigma, strain on families and health systems	~13 million AIDS orphans in sub-Saharan Africa
Economic	Lost workforce, healthcare costs, deterred investment/tourism	Malaria costing sub-Saharan Africa billions/year
LIC vs HIC	Same disease far worse in LICs (weak systems, poverty)	Cholera trivial in HICs, deadly in slums/camps

The take-home for 14.2.2: pathogenic disease has DEMOGRAPHIC, SOCIAL and ECONOMIC impacts that reinforce one another, and because they fall hardest on poor countries with weak health systems, disease both reflects and deepens global inequality and acts as a major brake on development.