AP Syllabus focus:
‘SARS can spread through inhaling or touching infected fluids; MERS is transferred from animals to humans. Zika spreads via mosquitoes and sexual contact, and cholera is contracted from infected water.’
Emerging and re-emerging infectious diseases can spread rapidly when environmental conditions, human behaviour, and infrastructure allow transmission.
The chain-of-infection model shows the key steps required for transmission: an infectious agent, a reservoir, portals of exit and entry, a mode of transport, and a susceptible host. In environmental science terms, prevention works by ‘breaking’ one or more links—e.g., ventilation and surface hygiene for SARS, vector control for Zika, and safe water/sanitation for cholera. Source
SARS, MERS, Zika, and cholera illustrate key pathways: airborne/contact, zoonotic spillover, vector-borne/sexual, and waterborne.
Core ideas for AP Environmental Science
These four diseases are useful examples because they connect pathogen transmission to environmental systems:
Built environments (crowding, ventilation, healthcare settings)
Animal–human interfaces (livestock markets, farming, wildlife contact)
Vector ecology (mosquito habitat shaped by climate and water storage)
Water and sanitation (drinking-water quality, wastewater management)
Key transmission terms
Zoonotic disease: an infectious disease that can be transmitted from non-human animals to humans.
Zoonotic spillover risk increases when humans and animals interact more frequently or intensely, especially where biosecurity is limited.
Vector: an organism (often an arthropod such as a mosquito) that transmits a pathogen between hosts.
Vector populations are strongly influenced by temperature, precipitation, and the availability of standing water.
SARS (Severe Acute Respiratory Syndrome)
How it spreads (spec-aligned)
Inhaling infected fluids: exposure to respiratory droplets produced by coughing or sneezing, especially in close-contact indoor settings.
Touching infected fluids: contact with contaminated secretions on hands or surfaces followed by touching the mouth, nose, or eyes (a contact/fomite route).
Environmental and systems factors that influence outbreaks
Indoor air and crowding: poorly ventilated indoor spaces can increase exposure to infectious droplets.
Healthcare settings: high contact rates, aerosol-generating procedures, and inadequate protective equipment can amplify spread.
Mobility and connectivity: modern travel networks can move infected individuals between cities before detection.
High-utility prevention links
Ventilation and filtration in shared indoor spaces
Surface hygiene and handwashing infrastructure in public and clinical settings
Isolation protocols that reduce contact rates during infectious periods
MERS (Middle East Respiratory Syndrome)
How it spreads (spec-aligned)
Transferred from animals to humans: a zoonotic transmission pathway (commonly discussed in terms of camel-to-human spillover in affected regions).
Environmental and land-use context
Animal husbandry and trade: close daily contact with domesticated animals increases opportunities for cross-species transmission.
Biosecurity: inadequate separation of animal housing, food handling, and human living/working spaces can increase exposure.
Occupational risk: farm workers, animal market workers, and some healthcare workers may have elevated risk due to frequent contact.
Control strategies tied to environmental management
Reduce high-risk animal–human contact through farm hygiene, controlled animal movement, and safer handling of animal products.
Strengthen surveillance where spillover is most likely (farms, markets, transport hubs).
Zika
How it spreads (spec-aligned)
Via mosquitoes: transmission by Aedes mosquitoes, which often breed in small containers of water near human habitation.
Diagram of the Aedes mosquito life cycle (eggs, larva, pupa, adult), emphasizing that immature stages develop in water and adults emerge to bite hosts. This supports why eliminating small standing-water habitats can sharply reduce vector abundance and interrupt Zika transmission.
Sexual contact: human-to-human transmission independent of vectors, allowing spread even where mosquitoes are absent.
Environmental drivers of mosquito-borne spread
Standing water: buckets, tires, clogged drains, and uncovered water storage create breeding sites.
Climate sensitivity: warmer temperatures can expand mosquito habitat suitability and lengthen transmission seasons.
Urban infrastructure: inconsistent piped water can increase household water storage, unintentionally supporting mosquito breeding.
Prevention strategies with environmental relevance
Source reduction: eliminate breeding sites by draining containers and improving waste management that prevents water accumulation.
Barriers: window screens, bed nets where appropriate, and targeted repellents.
Public health integration: sexual-transmission risk reduction messaging complements vector control (important during pregnancy planning due to fetal risks).
Cholera
How it spreads (spec-aligned)
Contracted from infected water: ingestion of water contaminated with the cholera bacterium, typically via faecal contamination.
The F-diagram maps the major fecal–oral pathways by which pathogens move from feces to a new host—especially via contaminated water and hands, but also via food and flies. The vertical barrier lines highlight how sanitation, safe water, and hygiene interventions block these routes, which is central to understanding cholera prevention through environmental infrastructure. Source
Environmental conditions that increase risk
Poor sanitation: inadequate sewage treatment or open defecation can introduce pathogens into surface water and shallow groundwater.
Unsafe drinking-water systems: insufficient treatment, broken pipes, or intermittent service that allows contamination.
Flooding and displacement: disasters can overwhelm sanitation systems and force reliance on contaminated sources.
Coastal and estuarine dynamics: contamination can spread through connected waterways used for washing, cooking, or drinking.
Prevention grounded in environmental infrastructure
Safe water: reliable treatment (e.g., filtration/disinfection) and protected distribution systems.
Sanitation: effective waste containment and treatment to prevent water contamination.
Rapid response: emergency water provision and hygiene resources in displaced communities.
FAQ
Aedes often breed in small, human-made water containers.
Key contributors include:
Intermittent water supply leading to household water storage
Poor solid-waste management (discarded plastics/tyres that trap rainwater)
Clogged drains and standing water from inadequate maintenance
These conditions can concentrate mosquito breeding close to people, increasing bite rates.
It adds a non-vector pathway, so transmission can continue even when mosquito numbers are low.
Responses may therefore include:
Guidance on condom use and timing of conception after possible exposure
Testing and counselling for travellers returning from affected areas
This is especially important where pregnancy outcomes are a primary concern.
Risk rises when exposure frequency and pathogen shedding opportunities increase.
Examples of higher-risk interfaces:
Dense animal housing with limited hygiene controls
Markets with many animals from different sources
Handling of animal secretions during milking, slaughter, or veterinary care
Reducing exposure opportunities is often as important as medical treatment.
Indoor environments can increase exposure when infectious droplets accumulate or when close-contact interactions are frequent.
Mitigation often focuses on:
Increasing outdoor air exchange
Improving filtration where recirculated air is used
Reducing crowding during outbreaks
These are environmental controls that complement behavioural measures like isolation.
Flooding can mix sewage with drinking-water sources and damage treatment and distribution systems.
Displacement can worsen risk by:
Increasing crowding and shared sanitation
Creating reliance on temporary water supplies
Reducing capacity for safe waste disposal
Rapid provision of safe water and sanitation facilities is critical in these settings.
Practice Questions
State two distinct transmission routes for Zika virus described in the specification. (2 marks)
1 mark: transmitted via mosquitoes (Aedes mosquito vector).
1 mark: transmitted via sexual contact.
Explain how environmental conditions and infrastructure can increase the risk of cholera outbreaks, and outline practical environmental interventions that reduce transmission. (6 marks)
1 mark: cholera is contracted from infected (contaminated) water.
1 mark: poor sanitation/sewage contamination introduces pathogens into water supplies.
1 mark: unsafe or intermittent drinking-water systems/pipes allow contamination of distributed water.
1 mark: flooding/disasters/displacement can overwhelm sanitation and increase reliance on unsafe water.
1 mark: intervention—improve water treatment and protect water distribution (e.g., disinfection and secure pipes).
1 mark: intervention—improve sanitation/wastewater containment and treatment to prevent faecal contamination.
