AP Syllabus focus:
‘Plague can be transferred by bites from infected organisms or contact with contaminated tissues. Tuberculosis spreads via inhaling bacteria; malaria and West Nile virus are transmitted by infected mosquitoes.’
These four diseases illustrate major pathways of infectious disease spread: animal-to-human contact, airborne transmission, and insect vectors. Environmental conditions, human behaviour, and land-use can increase exposure and shape outbreak risk.
Core ideas for AP Environmental Science
Pathways of transmission
Many infectious diseases depend on how a pathogen moves between hosts and persists in the environment.
Pathogen: A disease-causing organism or infectious agent (such as a bacterium, virus, or parasite).
Transmission can involve direct contact, air, or other organisms that carry pathogens.
Vector: An organism (often an insect) that transmits a pathogen from one host to another.
Environmental controls on disease risk
Key environmental factors that influence these examples include:
Habitat conditions that support hosts or vectors (e.g., rodent habitat, mosquito breeding sites)
Water management (standing water increases mosquito reproduction)
Sanitation and housing quality (affects exposure to rodents, contaminated tissues, and indoor air)
Human movement and land use (trade, urbanisation, irrigation, deforestation can shift contact rates)
Plague
How plague spreads (as required)
Plague can be transferred by bites from infected organisms or by contact with contaminated tissues.
CDC illustration of plague ecology, showing Yersinia pestis cycling among rodents and fleas and the pathways that lead to human exposure. The figure visually links reservoir hosts (rodents), vectors (fleas), and spillover to humans and domestic animals, aligning with zoonotic transmission concepts. Source
The classic cycle involves rodents (reservoir hosts) and fleas (vectors), with humans as incidental hosts.
Environmental and human drivers
Rodent population dynamics: Food availability (waste, grain storage) and mild conditions can raise rodent numbers, increasing human–rodent contact.
Built environment: Poorly sealed buildings and unmanaged refuse provide shelter and food for rodents.
Occupational exposure: Hunting, trapping, butchering, and handling carcasses increase risk from contaminated tissues.
Public health link: Reducing exposure often relies on rodent control, safe waste storage, and minimising direct handling of potentially infected animals.
Tuberculosis (TB)
How TB spreads (as required)
Tuberculosis spreads via inhaling bacteria. When an infected person coughs, tiny respiratory particles can remain suspended and be inhaled by others, especially in enclosed spaces.
Environmental and social context
Indoor air and crowding: Poor ventilation and high occupancy increase the chance of inhalation exposure.
Risk settings: Shelters, prisons, and crowded housing can elevate transmission due to sustained close contact.
Environmental health focus: While TB is not “waterborne,” it is strongly influenced by housing quality, ventilation, and access to healthcare that reduces the time a person remains infectious.
Malaria
How malaria spreads (as required)
Malaria is transmitted by infected mosquitoes.
CDC schematic of the malaria life cycle, showing transmission by female Anopheles mosquitoes and the parasite’s major stages in humans (liver and red blood cells). The labeled cycle clarifies why mosquito exposure is the key control point for preventing malaria transmission. Source
Mosquitoes act as vectors, transferring the parasite between humans during blood feeding.
Environmental conditions that increase mosquito-borne transmission
Standing water: Puddles, ditches, containers, irrigated fields, and poorly drained areas can become breeding habitat.
Water projects and agriculture: Irrigation and reservoirs can expand breeding sites if not managed with vector control in mind.
Land-use change: Deforestation and settlement patterns can increase human contact with mosquito habitat.
Exposure timing and protection: Transmission risk increases when people are outdoors or unprotected during peak mosquito activity.
West Nile virus
How West Nile spreads (as required)
West Nile virus is transmitted by infected mosquitoes.
CDC diagram of the West Nile virus transmission cycle, emphasizing the mosquito–bird amplification loop and spillover infections in humans and horses. The labeled arrows make it easy to distinguish reservoir hosts (birds) from incidental, dead-end hosts (humans), a key concept in environmental disease ecology. Source
Many transmission cycles involve mosquitoes and bird hosts, with humans infected incidentally.
Environmental and community risk factors
Urban stormwater features: Catch basins, clogged gutters, and unmanaged containers can produce breeding sites.
Heat and drought patterns: Local conditions that concentrate birds and mosquitoes or speed mosquito development can raise risk (mechanisms vary by region).
Prevention emphasis: Community-level mosquito surveillance, habitat reduction (source control), and personal protective actions reduce exposure.
Comparing the four examples (high-utility distinctions)
Plague: often zoonotic exposure via fleas/rodents or contaminated animal tissues.
Zoonotic disease: A disease that can spread from non-human animals to humans.
Zoonoses commonly intensify where wildlife, domestic animals, and humans overlap.
TB: primarily airborne inhalation in indoor environments; strongly shaped by ventilation and crowding.
Malaria & West Nile: vector-borne; risk rises with mosquito habitat availability and human–mosquito contact.
FAQ
Species matters because breeding habitat and biting times differ.
Malaria is mainly spread by Anopheles mosquitoes.
West Nile is commonly spread by Culex mosquitoes.
Animal reservoirs can maintain the pathogen at low levels.
When rodent and flea populations rise, “spillover” to humans becomes more likely.
TB is airborne rather than waterborne.
Control depends heavily on ventilation, early diagnosis, and reducing time spent infectious in enclosed spaces.
Birds can act as amplifying hosts for the virus.
Local bird community composition and roosting patterns can affect mosquito infection rates.
Reducing breeding sites can cut mosquito populations at the source.
This can lower reliance on chemicals and slow the development of insecticide resistance.
Practice Questions
State two different transmission routes mentioned in the syllabus excerpt for the diseases listed. (2 marks)
Any two correctly stated routes (1 mark each), e.g. bites from infected organisms; contact with contaminated tissues; inhaling bacteria; transmission by infected mosquitoes.
Explain how environmental conditions can increase the spread of (i) malaria and (ii) tuberculosis, using the transmission routes given. (6 marks)
Malaria: transmitted by infected mosquitoes (1)
Link to environment: standing water provides breeding habitat (1)
Link to human/environment interaction: irrigation/poor drainage/containers increase standing water (1)
TB: spreads via inhaling bacteria (1)
Link to environment: crowding increases exposure to exhaled bacteria (1)
Link to environment: poor ventilation increases persistence/inhalation risk indoors (1)
