AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/8/8-15)
Unit 8 · Topic 8.15
8.15 Pathogens and Infectious Diseases
Pathogens are disease-causing organisms such as viruses, bacteria, protists and parasitic worms. Many spread through contaminated water or via vectors such as mosquitoes, and poverty, crowding, poor sanitation, travel and climate change all help them spread.
Key terms
- pathogen
- vector
- malaria
- tuberculosis
- cholera
Pathogens and how they spread
A pathogen is any living thing or virus that causes disease: viruses, bacteria, protists (single-celled organisms like the one that causes malaria) and parasitic worms. An infectious disease is one caused by a pathogen that can spread from one host to another.
A vector is a living thing, often an insect, that carries a pathogen from one host to another. Mosquitoes, fleas and ticks are common vectors. Other diseases spread through contaminated water or food, through the air when people cough, or by close contact.
What helps diseases spread
- Poor sanitation and lack of clean water: waterborne diseases like cholera thrive where sewage mixes with drinking water. Developing countries carry much of this burden.
- Crowding and urbanization: dense populations let airborne and contact diseases pass quickly.
- Travel and trade: an infected person or mosquito can cross the world in a day by plane or ship.
- Climate change: warmer temperatures let mosquitoes and ticks survive farther from the equator and at higher elevations, extending the range of diseases like malaria and dengue (topic 9.4).
- Contact with wildlife: clearing forests and wildlife trade bring people into contact with animals that carry new pathogens.
- Pathogen adaptation: pathogens evolve, for example by becoming resistant to antibiotics, or by adapting to new hosts.
Diseases to know
| Disease | Pathogen | How it spreads | Notes |
|---|---|---|---|
| Malaria | Plasmodium (a protist) | Bite of infected Anopheles mosquitoes | Kills hundreds of thousands a year, mostly young children in sub-Saharan Africa |
| Tuberculosis | Bacterium | Through the air when infected people cough | Attacks the lungs; drug-resistant strains are a growing problem |
| Cholera | Bacterium | Water or food contaminated with feces | Severe watery diarrhea and dehydration; outbreaks follow disasters that damage water systems |
| Plague | Bacterium | Bites of fleas that live on rodents | Caused the Black Death; still occurs in a few places today |
| West Nile virus | Virus | Mosquito bites; birds are the main hosts | Reached the United States in 1999 and spread across the country |
| Zika | Virus | Aedes mosquito bites, sexual contact, and from mother to fetus | Can cause severe birth defects such as microcephaly (an unusually small head) |
| SARS | Coronavirus | Respiratory droplets and close contact | Outbreak in 2002–2003, starting in southern China |
| MERS | Coronavirus | Close contact, including with camels | First identified in the Middle East in 2012; high death rate |
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Reasoning chain: climate and malaria
Explain how a warming climate could increase malaria cases in a highland region of East Africa where malaria has historically been rare.
Show the solutionHide the solution
- Step 1: Malaria is spread by Anopheles mosquitoes, and both the mosquito and the Plasmodium parasite develop faster in warmer temperatures.
- Step 2: Highland areas used to be too cool for mosquitoes and the parasite to thrive, which kept malaria rare.
- Step 3: As average temperatures rise, conditions at higher elevations become warm enough for mosquito populations to grow and for the parasite to complete its life cycle inside them.
- Step 4: People in these areas have little immunity because they haven't been exposed before, so cases and deaths can rise quickly.
Answer: Warming lets the mosquito vector and the parasite survive at higher elevations, exposing people with little immunity, so malaria cases rise.
- Example 2
Trap: pathogen versus vector
A student writes: 'Mosquitoes cause malaria.' Rewrite the statement accurately.
Show the solutionHide the solution
- Step 1: The pathogen is the organism that causes the disease: the Plasmodium protist.
- Step 2: The mosquito is the vector: it carries the parasite from person to person through its bite.
- Step 3: So the mosquito spreads the disease, while the parasite causes it.
Answer: Malaria is caused by the Plasmodium parasite and spread by Anopheles mosquitoes, which act as the vector.
Common mistakes
- Calling the vector the pathogen. Mosquitoes carry malaria; Plasmodium causes it.
- Calling malaria a virus or bacterial disease. It is caused by a protist.
- Giving 'poor hygiene' as the reason for every disease. Tuberculosis is airborne, malaria is vector-borne, and cholera is waterborne; match the cause to the route.
On the exam
- Expect to match a disease to its pathogen type or its mode of transmission, or to explain how climate change could affect a vector-borne disease.
- When asked how to reduce a disease, choose a solution that fits its route: clean water and sanitation for cholera, bed nets and draining standing water for malaria.
Connected topics
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Check yourself
4 questions on 8.15 Pathogens and Infectious Diseases. Pick an answer to see if you got it, and why.
As average temperatures rise, malaria has been reported at higher elevations in some tropical mountain regions where it was once rare. Which explanation is best supported?
After a flood damages a city's water and sewer systems, an outbreak of severe watery diarrhea is traced to a bacterium in the drinking water. Which disease is the most likely cause?
Which condition most increases the spread of tuberculosis, a bacterial disease of the lungs?
Which pairing of disease and how it is spread is correct?
0 of 4 answered