Pathogens &
Infectious Diseases

Explore how bacteria, viruses, prions, and parasites cause disease, how they spread through populations, and why environmental factors shape emerging infectious diseases.

AP Environmental Science

What Is a Pathogen?

A pathogen is any biological agent that causes disease in a host organism. Pathogens exploit the host's resources to reproduce, often damaging tissues and triggering immune responses in the process.

In environmental science, understanding pathogens matters because human activity: deforestation, urbanization, climate change, and global trade: creates new opportunities for pathogens to jump between species and spread across the globe.

Four Major Pathogen Types

Bacteria

Single-celled prokaryotes. Some produce toxins; others invade tissues directly. Treatable with antibiotics.

Viruses

Non-living particles of DNA or RNA in a protein coat. Require a host cell to replicate. Fought with vaccines.

Prions

Misfolded proteins that cause other proteins to misfold. No DNA or RNA. Currently untreatable.

Parasites

Eukaryotic organisms (protists, worms, insects) that live on or in a host. Include malaria and tapeworms.

How Infectious Diseases Spread

Explain what fecal indicators can and cannot establish.

Use this illustration to explore the concept described in its caption.

A gloved hand holds a water sample beside a stream. An indicator detected in the sample leads to an investigation, while pathogens remain uncounted and the source is unestablished. — A positive indicator result supports investigating possible fecal contamination; it does not enumerate pathogens or identify the source.

Environmental exposure routes are pathogen-specific

Distinct windows show an outfall/water-contact scene and an enlarged mosquito near standing water. No larval counts or actual disease diagnosis; mosquito enlargement is illustrative. Separate exact panel: Three separate rows trace sewage through water or food to ingestion, infected hosts through mosquitoes to bites, and snail-associated freshwater exposure to skin contact.

Environmental exposure pathways depend on the pathogen. Schistosoma includes a freshwater snail stage and can enter through skin, while mosquito-borne transmission and contaminated drinking water require different interruption strategies.

Ingestion, insect-vector transmission and skin penetration are distinct exposure pathways.

Environmental exposure pathways depend on the pathogen. Schistosoma includes a freshwater snail stage and can enter through skin, while mosquito-borne transmission and contaminated drinking water require different interruption strategies.

Endemic vs Epidemic vs Pandemic

Endemic

A disease that is constantly present at a baseline level in a specific geographic area. Example: malaria in sub-Saharan Africa.

Epidemic

A sudden increase in cases beyond the expected level in a region. Example: the 2014 Ebola outbreak in West Africa.

Pandemic

An epidemic that has spread across multiple countries or continents. Example: COVID-19 beginning in 2020.

The Disease Triangle

Virulence and Dose-Response

Virulence measures how severe a pathogen's effects are. A highly virulent pathogen causes serious illness at low doses, while a less virulent one requires larger exposure. The infectious dose is the minimum number of pathogen units needed to establish infection.

Bacteria vs Viruses

	Bacteria	Viruses
Cell structure	Living, single-celled prokaryote	Non-living; protein coat + nucleic acid
Reproduction	Binary fission (independent)	Hijacks host cell machinery
Treatment	Antibiotics	Antivirals / Vaccines
Size	~1,000 nm	~20 to 300 nm
Examples	Cholera, TB, E. coli	Influenza, HIV, Ebola

Emerging Infectious Diseases

An emerging infectious disease (EID) is one that has recently appeared in a population, is rapidly increasing in incidence, or is threatening to increase. Most EIDs are zoonotic: they originate in animal populations and jump to humans.

1

Deforestation forces wildlife into closer contact with humans, increasing spillover events.

2

Climate change expands the habitat range of disease vectors like mosquitoes and ticks.

3

Urbanization creates dense populations where airborne and waterborne diseases spread quickly.

4

Global travel and trade move pathogens across continents in hours, outpacing containment efforts.

Environmental Drivers of Disease Emergence

Analyzing an Outbreak: Step by Step

1

Identify the pathogen. Determine if the causative agent is a bacterium, virus, prion, or parasite.

2

Determine the transmission route. Is it direct contact, vector-borne, waterborne, airborne, or foodborne?

3

Assess environmental factors. What human activities or environmental conditions enabled the outbreak?

4

Evaluate population vulnerability. Consider population density, immune status, access to healthcare, and sanitation infrastructure.

5

Propose interventions. Recommend prevention strategies: vaccination, vector control, sanitation, quarantine, or behavior change.

Example: Cholera Outbreak

Scenario: After severe flooding in a low-income coastal city, hundreds of residents develop acute watery diarrhea. Water treatment facilities were damaged by the flood.

1

Pathogen: Vibrio cholerae, a bacterium.

2

Transmission: Waterborne: contaminated floodwater mixed with sewage enters drinking supplies.

3

Environmental factors: Flooding, inadequate sanitation, damaged infrastructure, climate-driven extreme weather events.

4

Vulnerability: Dense population, limited healthcare access, no prior vaccination.

5

Interventions: Oral rehydration therapy, water purification tablets, restore treatment plants, oral cholera vaccine campaign.

Example: Malaria Expansion

Scenario: A highland region in East Africa that historically had no malaria cases begins reporting infections. Average temperatures in the region have risen by \(1.5^\circ\text{C}\) over the past two decades.

Pathogen

Plasmodium falciparum, a protozoan parasite transmitted by Anopheles mosquitoes.

Climate Link

Warmer temperatures allow mosquitoes to survive at higher altitudes, expanding their range into previously cooler highlands.

Example: Antibiotic Resistance

Scenario: A hospital reports that Staphylococcus aureus infections are no longer responding to standard antibiotics. The surrounding agricultural region uses large quantities of antibiotics in livestock feed.

Common Mistakes

✕ Antibiotics kill viruses

Antibiotics only work against bacteria. Viruses require antivirals or prevention through vaccines.

✕ All microorganisms are harmful

Most microbes are benign or beneficial. Only those causing host damage are classified as pathogens.

✕ Epidemic = pandemic

An epidemic is regional. A pandemic crosses international borders and affects multiple continents.

Pathogen Summary Table

	Bacteria	Viruses	Prions	Parasites
Living?	Yes	No	No	Yes
Genetic material	DNA	DNA or RNA	None	DNA
Reproduction	Binary fission	Host cell	Misfolding	Sexual/asexual
Treatment	Antibiotics	Antivirals	None	Antiparasitics
Example	Cholera	Influenza	Mad cow (BSE)	Malaria

Question 1 of 5

Which type of pathogen consists of misfolded proteins and contains no genetic material (DNA or RNA)?

Prions are unique because they are misfolded proteins that cause other proteins to misfold. Unlike bacteria, viruses, and parasites, they contain absolutely no nucleic acid.

Question 2 of 5

A disease that is constantly present at a baseline level in a specific geographic region is best described as:

Endemic describes a disease that persists at a predictable baseline level in a region. An epidemic is a sudden spike above that baseline, and a pandemic is an epidemic that crosses international borders.

Question 3 of 5

Climate change contributes to the spread of malaria primarily by:

Warmer temperatures allow Anopheles mosquitoes (the vector for Plasmodium) to survive and breed at higher altitudes and latitudes that were previously too cold, introducing malaria to new populations with no immunity.

Question 4 of 5

Observation: A coastal city experiences a major cholera outbreak after a hurricane damages water treatment infrastructure. Investigators find that Vibrio cholerae concentrations in the city's water supply are several hundred times higher than safe limits. Neighboring inland cities that share the same regional food supply but have intact water systems report no cholera cases.

▼ Tap to read passage

Based on the information above, which vertex of the Disease Triangle was most directly altered to cause this outbreak?

The key evidence is that inland cities with intact water systems had no cases, despite sharing the same food supply and pathogen exposure risk. The hurricane changed the environmental vertex by destroying water treatment, allowing the pathogen to reach hosts through contaminated water.

Question 5 of 5

Antibiotic resistance in bacteria is best explained by which biological mechanism?

Antibiotic resistance is a textbook example of natural selection. When antibiotics are applied, susceptible bacteria die, but those with pre-existing resistance genes survive, reproduce, and pass those genes to the next generation: and even to other species via horizontal gene transfer.

Pathogens &Infectious Diseases

What Is a Pathogen?

Four Major Pathogen Types

How Infectious Diseases Spread

Explain what fecal indicators can and cannot establish.

Environmental exposure routes are pathogen-specific

Ingestion, insect-vector transmission and skin penetration are distinct exposure pathways.

Endemic vs Epidemic vs Pandemic

The Disease Triangle

Virulence and Dose-Response

Bacteria vs Viruses

Emerging Infectious Diseases

Environmental Drivers of Disease Emergence

Analyzing an Outbreak: Step by Step

Example: Cholera Outbreak

Example: Malaria Expansion

Example: Antibiotic Resistance

Common Mistakes

Pathogen Summary Table

Question 1 of 5

Question 2 of 5

Question 3 of 5

Question 4 of 5

Question 5 of 5

Pathogens &
Infectious Diseases