AP Environmental Science

## Learning Outcomes

- Explain why malaria is a **vector-borne disease** rather than a directly contagious disease
- Identify the roles of the **Plasmodium** parasite and the female **Anopheles** mosquito
- Describe the basic malaria transmission cycle from mosquito to human and back again
- Explain how temperature, rainfall, humidity, altitude, and standing water affect malaria distribution
- Distinguish between **endemic** and **epidemic**, and between **incidence** and **prevalence**
- Recall the main global pattern of malaria burden, especially the concentration in sub-Saharan Africa
- Apply malaria knowledge to AP Environmental Science data, map, and FRQ-style questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand malaria transmission and distribution, with a focus on the following syllabus points:

- Know that malaria is caused by a **parasite** and transmitted mainly by infected female **Anopheles** mosquitoes
- Identify malaria as a **vector-borne infectious disease**, not a disease spread by ordinary person-to-person contact
- Explain the basic transmission cycle: mosquito bite, liver stage, blood stage, and return to mosquito
- Understand how temperature, rainfall, humidity, altitude, and standing water affect mosquito survival and parasite development
- Know that malaria is concentrated in tropical and subtropical regions, especially sub-Saharan Africa
- Explain how land-use changes such as irrigation, poor drainage, rice paddies, deforestation, and mining pits can increase mosquito breeding habitat
- Distinguish **endemic** from **epidemic**
- Distinguish **incidence** from **prevalence** when interpreting malaria data

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. Why is malaria classified as a vector-borne disease?
2. What is the difference between the malaria pathogen and the malaria vector?
3. How do temperature and rainfall affect malaria transmission?
4. Why is malaria much more common in some parts of the world than in others?
5. What is the difference between incidence and prevalence in malaria data?

## Introduction

Malaria is one of the most important infectious diseases in AP Environmental Science because it links biology, climate, land use, water conditions, and public health. It is not just a medical issue. It is also an environmental issue because malaria spreads only where ecological conditions allow the mosquito vector to survive and the parasite to complete its life cycle.

For exam purposes, you need to understand more than the fact that mosquitoes are involved. You need to know which mosquito, what pathogen is transmitted, why distribution is uneven across the world, and how environmental conditions change disease risk. This makes malaria a strong example of how human health depends on environmental systems.

> **Key Term: pathogen**
> A disease-causing organism or agent that infects a host.

### Malaria as a vector-borne disease

Malaria is caused by **Plasmodium** parasites and is transmitted mainly through the bites of infected female **Anopheles** mosquitoes. That wording matters. Students often say “mosquitoes spread malaria,” which is too vague for the exam. Not all mosquitoes transmit malaria, and the expected vector is the female _Anopheles_ mosquito.

> **Key Term: vector**
> An organism that carries and transmits a pathogen from one host to another.

The mosquito is the **vector**, not the pathogen. The **Plasmodium** organism is both the pathogen and a parasite. This is a common AP trap: mixing up the carrier with the disease-causing organism.

> **Key Term: parasite**
> An organism that lives in or on a host and benefits at the host’s expense.

Malaria is called **vector-borne** because the parasite usually needs the mosquito to move between human hosts. It is not usually spread by casual human contact. You do not get malaria from sitting next to an infected person, sharing a room, coughing, or touching surfaces. That makes malaria very different from airborne diseases such as influenza or SARS.

Rare non-mosquito transmission can happen through blood transfusion, shared contaminated needles, organ transplantation, or mother-to-child transmission. But these are exceptions, not the main route.

Why does this distinction matter? Because if a question asks how to reduce malaria, the correct focus is often vector control or habitat management, not isolation of infected people. Since the mosquito is central to transmission, the environment around the mosquito becomes central to prevention.

> **Test Tip:** If an AP question asks for the “pathogen,” answer **Plasmodium**. If it asks for the “vector,” answer infected female **Anopheles** mosquito. Do not swap them.

### The malaria transmission cycle and why the environment matters

At AP level, you only need a simple but accurate life cycle. An infected female _Anopheles_ mosquito bites a human and injects parasite forms called sporozoites. These first travel to the **liver**. After multiplying there, the parasites enter the bloodstream and infect **red blood cells**. This blood-stage infection causes the main symptoms of malaria, such as fever, tiredness, vomiting, and headache. Some parasites then develop into forms that another mosquito can ingest during a later blood meal. Inside that mosquito, the parasite develops further, and the cycle continues.

This matters environmentally because malaria transmission depends on two linked biological processes happening successfully:

1. The mosquito must survive in that habitat.
2. The parasite must complete development inside the mosquito.

If either process fails, transmission drops. A region may have humans and standing water, but if temperatures are too low for the parasite to develop in the mosquito, malaria transmission may still be minimal. This is why malaria distribution is not random.

A useful way to think about malaria is as a chain:

- standing water or suitable breeding sites increase mosquito larvae
- more adult female mosquitoes increase biting frequency
- warm enough temperatures speed parasite development in the mosquito
- higher humidity helps mosquitoes live longer
- longer mosquito survival increases the chance that the parasite matures and is passed on

That is the environmental logic behind malaria risk.

Some parasite species also differ. **Plasmodium falciparum** is the deadliest major human malaria parasite. **P. vivax** is also important and can relapse because some liver-stage forms can remain dormant. For AP Environmental Science, you do not need detailed medical treatment, but you should know that different parasite species can affect disease severity and persistence.

> **Key Term: vector-borne disease**
> A disease transmitted through a carrier organism rather than mainly by direct person-to-person spread.

### Why malaria occurs where it does

Malaria is concentrated in **tropical and subtropical** regions because these are the places where the mosquito vector and parasite are most likely to survive. The strongest global concentration is in **sub-Saharan Africa**, which has the overwhelming majority of cases and deaths. Textbook material emphasises that about 93% of malaria cases and deaths occur in sub-Saharan Africa, and current WHO reporting also shows that the African Region carries the vast majority of the burden.

This does not mean malaria is only African. It also occurs in parts of South Asia, Southeast Asia, Oceania, and parts of Central and South America. But the burden is far from evenly spread.

Why?

The first reason is **temperature**. Mosquitoes are ectothermic, so their activity, reproduction, and survival depend strongly on surrounding temperature. The parasite inside the mosquito also develops faster in warm conditions. If temperatures are too low, parasite development may slow or stop before the mosquito dies. That breaks transmission.

The second reason is **rainfall**. Mosquitoes need water to breed. Rain can create puddles, pools, ditches, and other larval habitats. In many regions, malaria transmission rises during or after rainy seasons. But rainfall is not a simple “more rain means more malaria” rule. Too little rain limits breeding sites, but extreme flooding can sometimes wash away larvae. The exam may reward this qualification.

The third reason is **humidity**. Adult mosquitoes survive better in humid air. Since a mosquito must live long enough for the parasite to develop inside it, humidity can strongly affect transmission success. Dry air shortens mosquito lifespan, reducing malaria spread even if temperatures are warm.

The fourth reason is **altitude**. Higher altitudes are generally cooler, which lowers mosquito survival and slows parasite development. This is why high-elevation areas often have less malaria than nearby lowlands. If climate warming raises temperatures in upland regions, those areas may become more suitable than before.

> **Key Term: endemic**
> A disease that is constantly present in a region at a baseline level.

Many malaria-affected places are described as **endemic**. That means malaria is regularly present there, not that it is rare. Students often get this wrong. If cases suddenly rise above the expected level in a place and time, that is an outbreak or epidemic.

> **Key Term: epidemic**
> A disease occurrence in which cases rise above the expected level in a given area and time period.

### Land use, breeding habitat, and AP Environmental Science links

Malaria belongs in Unit 8 because disease distribution is shaped by environmental conditions and human modification of land and water. Human activity often creates mosquito breeding habitat, even without intending to.

Common examples include:

- **irrigation** channels and poorly drained fields
- **rice paddies**, which provide shallow standing water
- **mining pits** that fill with water
- **deforestation**, which can change local shade, temperature, and water patterns
- **urban drainage failures** that leave stagnant water
- **water storage containers** in some communities
- **riparian stagnation** during dry conditions

The link to AP Environmental Science is cause and effect. If land use increases stagnant or slow-moving water, mosquito breeding may rise. If mosquito numbers rise, malaria risk may also rise, assuming the temperature and humidity remain suitable.

This helps explain why malaria can increase both in wetter conditions and in some drier conditions. In wetter areas, more rainfall can create more breeding pools. In drier areas, shrinking or stagnant water in riparian zones can become warm, shallow mosquito habitat. The key is not simply “wet” versus “dry.” The key is whether local conditions create breeding sites and allow adult mosquitoes to survive.

Climate change may intensify this in some places. Warmer temperatures can extend mosquito breeding seasons, reduce winter die-off, and increase the length of time in which transmission is possible. But again, this effect depends on local geography, moisture, and human habitat change.

> **Test Tip:** In FRQs, do not write “more rain always increases malaria.” A stronger answer says rainfall can create breeding sites, but too much flooding may wash away larvae, so the effect depends on local conditions.

### Interpreting malaria data: incidence, prevalence, endemicity, and burden

AP questions may use maps, tables, or case-study data. To handle these well, you need precise vocabulary.

> **Key Term: incidence**
> The number of new cases in a population over a given time period.
>
> **Key Term: prevalence**
> The total number of existing cases or infections in a population at a given time.

If a region reports 50 new malaria cases per 1,000 people at risk in one year, that is **incidence**. If a survey finds that 8% of children in a village currently have malaria infection, that is **prevalence**. Incidence helps show how quickly disease is spreading. Prevalence helps show how widespread it currently is.

These are different measures and answer different questions. Students often confuse them.

Malaria data are also usually reported for the **population at risk**, meaning the people living where transmission occurs. This is important because not everyone in the world is equally exposed. Malaria risk depends on environmental suitability, not simply total population size.

The global burden is still very high. WHO World Malaria Report 2024, reporting 2023 estimates, gave about **263 million cases** and **597,000 deaths** worldwide. The WHO African Region accounted for about **94% of cases** and **95% of deaths**. For AP purposes, the exact number matters less than the pattern: the burden is concentrated, not evenly distributed.

Why is that useful in an exam response? Because it lets you connect data to explanation:

- high burden in tropical regions
- strong concentration in sub-Saharan Africa
- environmental suitability for the vector
- limits posed by altitude, cold temperatures, or dry conditions elsewhere

A high-scoring response usually links geography to mechanism.

### Applying malaria to AP Environmental Science questions

Malaria may appear in multiple-choice or FRQ items asking you to connect disease to environmental conditions. A strong answer usually follows this chain:

- identify the vector and pathogen correctly
- explain the transmission route
- link transmission to temperature, rainfall, humidity, and habitat
- connect land use or climate change to breeding site availability
- support the explanation with geographic distribution

For example, if a question asks why malaria risk might increase in a newly irrigated farming district, a good answer would be:

- irrigation can create standing water
- standing water increases mosquito breeding habitat
- more female _Anopheles_ mosquitoes increase biting and transmission opportunities
- if temperatures are warm enough, the parasite develops successfully in mosquitoes
- therefore malaria risk increases

If a question asks why a high-elevation region has lower malaria prevalence than a nearby lowland plain, the logic is:

- higher altitude usually means cooler temperatures
- cooler temperatures reduce mosquito survival and slow parasite development
- fewer vectors survive long enough to transmit the pathogen
- so transmission is lower

That is the level of explanation AP Environmental Science rewards.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Malaria is caused by **Plasmodium** parasites
- Malaria is transmitted mainly by infected female **Anopheles** mosquitoes
- The mosquito is the **vector**; the parasite is the **pathogen**
- Malaria is usually not spread by ordinary person-to-person contact
- The basic cycle is mosquito bite to human liver to blood stage to mosquito
- Main malaria symptoms are caused during the **blood stage** in red blood cells
- Malaria is concentrated in tropical and subtropical regions
- The greatest global burden is in sub-Saharan Africa
- Temperature is a major control because both mosquito survival and parasite development depend on warmth
- Rainfall, humidity, and altitude strongly affect malaria transmission
- Human land-use changes such as irrigation, poor drainage, rice paddies, and mining pits can increase mosquito breeding habitat
- **Endemic** means regularly present; **epidemic** means above expected levels
- **Incidence** means new cases over time; **prevalence** means all existing cases at a given time

## Key Terms and Concepts

- pathogen
- vector
- parasite
- vector-borne disease
- endemic
- epidemic
- incidence
- prevalence


Pathogens and Infectious Diseases - Malaria transmission and distribution