AP Environmental Science

## Learning Outcomes

- Understand how **temperature, precipitation, and humidity** affect infectious disease spread
- Identify the difference between **vector-borne** and **waterborne** disease pathways
- Explain why tropical and subtropical regions often have longer transmission seasons
- Describe how climate warming can expand disease risk to higher latitudes and elevations
- Use examples such as **malaria, dengue, West Nile virus, Lyme disease, and cholera**
- Evaluate why climate is only one driver of disease risk, alongside sanitation, healthcare, and land use

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand climate zones and disease spread, with a focus on the following syllabus points:

- Know that **climate-sensitive infectious diseases** are influenced by weather and climate conditions
- Identify how **temperature, precipitation, and humidity** affect pathogen survival, vector abundance, and transmission risk
- Explain the difference between **vector-borne diseases** and **waterborne diseases**
- Understand why warmer, wetter conditions can increase mosquito breeding and bacterial activity in water
- Describe how climate warming can cause **geographic range expansion** into higher latitudes and higher elevations
- Recognise that tropical and subtropical regions often support longer or year-round transmission, while temperate regions are usually more seasonal
- Use examples including **malaria, dengue, West Nile virus, Lyme disease, cholera, dysentery, and zika**
- Understand that climate effects are regional and non-linear, not a guarantee of outbreaks everywhere

## 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. How do temperature and precipitation affect the spread of vector-borne disease?
2. What is the difference between a vector and a reservoir host?
3. Why are many infectious diseases more common in tropical and subtropical climate zones?
4. How can both flooding and drought increase disease risk?
5. Why does climate warming not automatically mean more disease in every region?

## Introduction

Climate zones shape where organisms can live, and that includes pathogens and the vectors that carry them. In AP Environmental Science, this topic links **pollution, climate, natural systemss, and human health**. You are not just learning disease names. You are learning how environmental conditions change transmission risk.

A strong exam answer usually follows a chain of reasoning: climate conditions alter habitat suitability, vector survival, or water quality; those changes alter human exposure; exposure changes disease incidence. The key is to explain the process, not simply state that “warming increases disease.”

> **Key Term: climate-sensitive infectious disease**
> A disease whose incidence or geographic distribution is influenced by weather and climate conditions.

## Introduction to climate control of disease

### The core climate drivers

Three climate variables matter most for disease spread: **temperature, precipitation, and humidity**. These do not affect all diseases in the same way, but they often control whether a pathogen or vector can survive long enough to infect hosts.

Temperature matters because it affects metabolism, development rate, and survival. Warmer conditions can lengthen the active season of mosquitoes and ticks, speed up reproduction, and in some cases speed pathogen development inside the vector. But there is a limit: extreme heat can also reduce survival.

Precipitation matters because water creates habitat. Mosquitoes often need standing water to breed. Heavy rain can also wash sewage, animal waste, and pathogens into rivers, lakes, or drinking water systems. Yet rainfall does not always raise disease risk. Strong floods can sometimes flush larvae away.

Humidity affects survival and behavior, especially for arthropod vectors such as mosquitoes and ticks. Dry air may reduce survival in some species, while moist air can help them remain active longer.

> **Key Term: vector**
> An organism that transmits a pathogen between hosts, such as a mosquito or tick.
>
> **Key Term: pathogen**
> A disease-causing agent such as a bacterium, virus, protozoan, fungus, or parasite.

A good APES connection here is the **Law of Tolerance**. Disease vectors have tolerance ranges just like other organisms. If climate conditions shift into a more favorable range, the vector’s geographic range may expand.

### Why climate zones matter

Tropical and subtropical climate zones are warm for much of the year and often humid. That means many pathogens and vectors face fewer seasonal limits. In these regions, mosquito populations can remain active for long periods or year-round, which increases opportunities for transmission.

Temperate climate zones usually have colder winters. These winters help restrict vector survival and shorten transmission seasons. Disease spread may still be serious, but it is more seasonal. This distinction matters on the exam: warmer tropical areas often support more continuous transmission, while temperate areas often show a spring-summer peak.

Cold climates generally restrict many insect vectors because freezing kills adults, larvae, or eggs, or stops their activity for long periods. If winters become milder, that barrier weakens.

Dry climates usually have fewer natural mosquito breeding sites, but disease risk does not disappear. People may store water in containers during drought, creating artificial breeding habitat. Dry conditions can also push humans and animals to share fewer water sources, raising exposure risk.

> **Test Tip:** If a question asks how climate zones affect disease spread, organise your answer around **temperature + moisture + season length**. That usually scores better than listing diseases without explanation.

## Vector-borne disease patterns across climate zones

### Tropical and subtropical diseases

Mosquito-borne diseases are the clearest examples of climate-sensitive infectious disease. The textbook notes that wetter conditions can increase breeding sites for mosquitoes, raising the rates of **malaria, dengue fever, zika, and yellow fever**. It also notes that warmer temperatures give insects longer breeding seasons, more reliable food access, and fewer winter die-offs.

Malaria is caused by a parasite spread by infected female _Anopheles_ mosquitoes. It is common in tropical and subtropical regions, especially sub-Saharan Africa. Why there? Because rainfall, humidity, and warm temperatures support mosquito survival and breeding, and also support development of the parasite inside the mosquito.

Dengue and zika are associated with mosquitoes in warm regions as well. Consistent warmth means the vectors remain active for more of the year. That does not mean every warm region will have equal disease burden. Urban density, sanitation, housing, and vector control all matter too.

This is a classic APES point: **climate creates suitability, but public health conditions influence actual disease outcomes**.

### Temperate diseases and seasonal spread

Temperate climates are not free from vector-borne disease. They usually show stronger seasonality. Two standard examples are **West Nile virus** and **Lyme disease**.

West Nile virus spreads through infected mosquitoes. Warmer temperatures and changing rainfall can increase mosquito abundance in temperate settings. If a warm season begins earlier and ends later, mosquitoes stay active for more time. That extends the transmission window.

Lyme disease is spread by ticks rather than mosquitoes. Ticks respond to temperature, moisture, and host availability. Milder winters and longer warm seasons can improve tick survival and lengthen their active period. This helps explain why tick range can shift northward.

> **Key Term: seasonality**
> The pattern of disease occurrence changing with the seasons, often because climate affects vectors, pathogens, or host exposure.

One distinction students often miss: West Nile and Lyme are both climate-sensitive, but they do not depend on the same vector or the same habitat. Mosquitoes need water for breeding. Ticks depend more on vegetation, humidity, and access to animal hosts.

### Range expansion to higher latitude and altitude

One of the most testable ideas is **geographic range expansion**. As average temperatures rise, places that were once too cold can become suitable for vectors and pathogens.

This can happen in two directions:

- toward **higher latitudes**, farther from the equator
- toward **higher elevations**, where cooler mountain climates become warmer

The textbook directly states that as equatorial-type climate zones spread north and south into subtropical and temperate zones, pathogens and vectors can spread into areas where they were not previously known.

A useful example is malaria risk in higher-elevation East African highlands. Another is the northward movement of ticks and mosquitoes in temperate regions.

> **Key Term: geographic range expansion**
> The spread of a species, vector, or disease risk into new areas where conditions have become suitable.
>
> **Test Tip:** On FRQs, do not say “climate change causes malaria.” Better wording is: “Climate change can increase habitat suitability for mosquitoes and lengthen transmission seasons, increasing malaria risk.”

## Waterborne disease, floods, drought, and climate variability

### Waterborne pathways are different from vector-borne pathways

A common exam mistake is mixing up **vector-borne** and **waterborne** disease. Cholera is not spread by mosquitoes. It is spread through contaminated water or food.

Flooding can raise waterborne disease risk because heavy rainfall can overwhelm sewage systems, carry untreated waste into water supplies, and increase runoff from farms or settlements. This contamination can spread pathogens such as those causing **cholera** and **dysentery**.

The textbook also notes that warmer water can increase bacterial activity in water supplies and may promote the spread of **amoebic dysentery, cholera, and giardia**.

> **Key Term: waterborne disease**
> A disease transmitted through contaminated water rather than by an animal vector.

Cholera is a strong APES example because it links climate, sanitation, and inequality. Outbreaks are closely tied to inadequate clean water and sewage infrastructure. Climate can worsen the risk, but poor sanitation is often the reason exposure actually occurs.

### Why both wet and dry conditions can increase disease risk

Students often assume that wetter conditions always mean more disease and drier conditions always mean less. That is too simple.

More rain can increase risk because it:

- creates standing water for mosquitoes
- causes sewage overflow
- contaminates rivers, lakes, and drinking water
- increases contact with polluted floodwater

But drought can also increase risk because it:

- causes water stagnation in some riparian areas
- raises water temperatures
- concentrates pollutants and pathogens in remaining water
- forces people to store water in containers, which can breed mosquitoes
- increases contact among humans, livestock, and wildlife at scarce water sources

The textbook explicitly notes that drier conditions can lead to stagnant water in riparian zones, higher water temperatures, higher bacterial counts, and increases in mosquito and other vector populations.

So the key pattern is not “wet equals disease.” It is **changed water conditions can increase transmission risk in different ways**.

### Climate variability and ENSO

Not every disease pattern comes from long-term climate change. Some come from **climate variability**, especially ENSO, the El Niño–Southern Oscillation.

ENSO shifts regional rainfall and temperature patterns on a cycle of about 2–7 years. It can change outbreak risk indirectly by altering:

- standing water availability
- drought severity
- flooding patterns
- water quality
- vector breeding conditions

ENSO does not directly cause disease. It changes environmental conditions that can favor outbreaks in some regions. That distinction is valuable in high-level answers.

## Why climate is not the only explanation

### Disease spread is multi-causal

Climate matters, but it is not enough by itself to explain disease burden. Two regions with similar temperatures may have very different disease outcomes because of differences in sanitation, housing, vector control, healthcare, and poverty.

Low-income areas often face higher infectious disease risk because they may lack:

- safe drinking water
- sewage treatment
- waste disposal systems
- screened housing
- reliable access to treatment

That is why cholera remains strongly linked to inequality, and why climate effects are often strongest where public health infrastructure is weakest.

> **Key Term: endemic**
> A disease consistently present at an expected level in a region.
>
> **Key Term: epidemic**
> A disease occurrence above the normal level in a region.
>
> **Key Term: pandemic**
> An epidemic that spreads across multiple countries or continents.

Climate may shift a disease from being absent to endemic, or may increase the chance of an epidemic, but the result depends on more than weather.

### Important distinctions and vocabulary

A vector is not the same thing as a **reservoir host**. A vector transmits the pathogen. A reservoir host maintains the pathogen in nature.

> **Key Term: reservoir host**
> The organism or environment in which a pathogen normally lives and is maintained.

Another key distinction is **incidence** versus **prevalence**. Incidence means new cases over a time period. Prevalence means all existing cases at a given time.

These terms help describe disease spread precisely. If climate warming extends transmission season, incidence may increase because there are more opportunities for new infections.

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

### A clear APES comparison table

| Disease         | Pathway                | Climate link                                                        | Typical climate pattern                             |
| --------------- | ---------------------- | ------------------------------------------------------------------- | --------------------------------------------------- |
| Malaria         | Vector-borne, mosquito | Rainfall, warmth, humidity affect mosquito and parasite development | Longer transmission in tropical/subtropical regions |
| Dengue          | Vector-borne, mosquito | Warmth and rainfall expand mosquito habitat                         | Common in tropical/subtropical zones                |
| West Nile virus | Vector-borne, mosquito | Temperature and precipitation affect mosquito abundance             | Often seasonal in temperate regions                 |
| Lyme disease    | Vector-borne, tick     | Milder winters and humidity affect tick survival                    | Seasonal, with warming-related range expansion      |
| Cholera         | Waterborne             | Flooding, warmer water, poor sanitation increase risk               | Strongly linked to contaminated water               |
| Dysentery       | Waterborne             | Contaminated water and sewage overflow increase spread              | Increased risk where sanitation fails               |

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Climate-sensitive infectious diseases are influenced by temperature, precipitation, and humidity
- Tropical and subtropical climate zones often support longer or year-round disease transmission
- Temperate climate zones usually show stronger disease seasonality because winter limits vectors
- Warmer temperatures can lengthen vector activity seasons and expand ranges into higher latitudes and elevations
- Mosquito-borne diseases include malaria, dengue, zika, and West Nile virus
- Tick-borne Lyme disease is affected by temperature, moisture, and host availability
- Waterborne diseases such as cholera and dysentery are linked to contaminated water, not vectors
- Flooding can increase sewage overflow and runoff contamination, raising waterborne disease risk
- Drought can also increase disease risk through stagnant water, unsafe storage, and concentrated pathogens
- Climate variability such as ENSO changes outbreak risk indirectly by shifting rainfall and temperature
- Climate does not act alone; sanitation, healthcare, poverty, housing, and vector control also affect disease burden
- Precise vocabulary matters, especially vector vs reservoir host and incidence vs prevalence

## Key Terms and Concepts

- climate-sensitive infectious disease
- vector
- pathogen
- seasonality
- geographic range expansion
- waterborne disease
- endemic
- epidemic
- pandemic
- reservoir host
- incidence
- prevalence


Pathogens and Infectious Diseases - Climate zones and disease spread