Pathogens & Infectious Diseases (College Board AP® Environmental Science): Study Guide

What are pathogens & infectious diseases?

• Pathogens are organisms that cause disease

  • These include bacteria, viruses, fungi, and parasites

  • They are found in water, air, food, soil, and other organisms

• Infectious diseases are illnesses caused by pathogens

  • They include diseases such as the flu, tuberculosis, cholera, and HIV

  • These diseases can be caused by genetics and the environment, and the two are often connected

• Diseases can devastate populations but do not usually drive their host to extinction because

  • Genetic diversity provides hosts with some natural resistance to pathogens

  • Pathogens can co-evolve with the host, which allows the host to adapt and resist the disease

  • Extinction of the host population jeopardizes the survival of the pathogen

  • After the initial infection of a host population, there are fewer opportunities for the disease to spread

How do pathogens cause disease? 

• Pathogens adapt to take advantage of new opportunities to infect and spread through human populations

• Pathogens enter the body through the skin, bodily fluids, or contaminated food, water, or soil

• They avoid the body's immune system and use the body's resources to replicate itself

• They then exit the body and spread to a new host

Occurrence of pathogens

• Pathogens can survive in many environments, even if those environments appear sterile

• In the past, infectious diseases had a high mortality rate

• Pathogens, like bubonic plague, influenza, and smallpox, have been the cause of some of the most significant outbreaks and pandemics in history

• Infectious diseases like SARS, MERS, and COVID-19 have brought widespread concern and attention in recent years

• There has also been a rise in drug-resistant bacteria and insecticide-resistant carriers, which, combined with overpopulation, exacerbate infectious diseases

• Overuse of antibiotics has allowed pathogens to develop drug resistance

  • TB was practically eradicated in much of the world, but drug-resistant strains have reversed that trend

  • DDT was commonly used to control malaria-carrying mosquitoes in tropical locations

    • After several years, mosquitoes developed a natural DDT resistance and have renewed the spread of the disease

    • Overprescription of anti-malarial drugs has also led to drug-resistant malaria

• Global travel has increased the likelihood of contracting or spreading infectious diseases

• The increase in population and urbanization has increased the spread and contraction of diseases from one another

• A lack of access to good health care increases the spread of infectious diseases

• Human behavior increases the risk of contracting and spreading an infectious disease such as:

  • refusing vaccinations

  • not using condoms

  • sharing hypodermic needles

  • refusing medical aid

• This indicates that pathogens still remain a very active threat to human health
  

Effect of global warming on pathogens & infectious diseases

• Global warming is expected to increase the spread and rate of infectious diseases by making it easier for disease-carrying vectors like mosquitoes and ticks to live

• This extends the transmission season and creates new environments that are favorable for pathogens to survive in

• This will increase the risk of:

  • outbreaks in places that were previously too cold

  • exposing more of the population to vector-borne diseases like malaria, dengue, and Lyme disease

  • changes in temperature and humidity affect pathogen survival and replication rates, making them more virulent and affecting host distribution and abundance

• Extreme weather-driven flooding can contaminate water supplies, raising the risk of waterborne illnesses including diarrhea and cholera

• Ecosystem adaptations to global warming could pave the way for the emergence of new and more deadly zoonotic diseases

Infectious diseases of poverty (IDoP)

• Poverty often creates conditions that easily spread disease

• Low-income areas often lack sanitary waste disposal and water supplies are contaminated

• This provides ideal conditions for the spread of infectious diseases

How poverty contributes to disease

• Poor living conditions and overcrowding can spread airborne diseases like tuberculosis and pneumonia 

• A lack of access to medical care and infrastructure can make it easier for diseases to spread 

• Job losses due to illness worsen poverty 

• Open fires or traditional stoves can lead to indoor air pollution and repiratory diseases 

• Lack of access to food, clean water, and sanitation can be fatal