Topic 8.15 Notes – Pathogens and Infectious Diseases

What Pathogens and Infectious Diseases Are

A pathogen is a disease-causing biological agent. In this topic, the main types are:

• Bacteria such as cholera, plague, and tuberculosis
• Viruses such as West Nile virus, SARS, MERS, and Zika
• Parasites such as malaria

An infectious disease happens when a pathogen enters a host, reproduces, and causes illness.

In APES, pathogens are treated like biological pollutants because they can move through water, air, waste, animals, and vectors and then harm human health. The point here is the environmental pathway of disease, not medical treatment or the cell biology of infection.

One detail students miss is that pathogens can be present even when water or surroundings look clean. They are microscopic, so appearance alone does not prove safety. Testing is often needed to detect them.

How Pathogens Move Through the Environment

Pathogens usually follow a cycle through the environment:

1. A reservoir holds the pathogen.
2. The pathogen leaves that source.
3. It moves through an environmental medium or a vector.
4. It enters a susceptible human host.
5. It reproduces in the host.
6. It is released back into the environment or passed to another host.

This cycle diagram also marks the points where control measures can interrupt transmission.

Reservoirs

A reservoir is where the pathogen lives or persists. Reservoirs can include:

• humans
• wildlife
• livestock or domesticated animals
• water
• soil
• waste

Vectors

A vector is a living organism that carries a pathogen between hosts.

• Important examples here are mosquitoes and fleas
• A vector is different from a passive medium like water

Main transmission routes

• Waterborne transmission through contaminated water
• Airborne or respiratory transmission through droplets or particles
• Vector-borne transmission through organisms like mosquitoes or fleas
• Zoonotic transmission from animals to humans
• Contact with contaminated tissues or bodily fluids
• Sexual transmission

Cholera as the clearest cycle

Cholera is the easiest example of the full cycle.

• An infected person releases bacteria in feces
• Sewage contaminates water
• Another person drinks that water
• That new infection adds more bacteria to the waste stream

Sanitation and water treatment break the cycle. In the diagram, those are examples of pathway control that stop the pathogen before it reaches the next host.

Required Diseases and Their Transmission Pathways

• Plague is a bacterial disease linked to rodents as reservoirs and fleas as vectors. It spreads by bite of an infected organism, especially a flea, or by contact with contaminated fluids or tissues.
• Tuberculosis is a bacterial lung infection spread by breathing in bacteria from respiratory fluids of an infected person. Crowding and poor ventilation increase spread.
• Malaria is a parasitic disease spread by infected mosquitoes. It is most common in sub-Saharan Africa.
• West Nile virus is a viral disease spread by infected mosquitoes. Birds are an important reservoir in the bird-mosquito cycle.
• SARS is a viral respiratory illness and a form of pneumonia. It spreads by inhaling or touching infected fluids.
• MERS is a viral respiratory illness transferred from animals to humans. Camels are the classic reservoir example.
• Zika is a viral disease spread by infected mosquitoes and also by sexual contact.
• Cholera is a bacterial disease contracted from infected or fecally contaminated water.

What Increases Disease Spread

Several environmental conditions create opportunities for disease.

• Pathogen adaptation means pathogens can take advantage of new hosts, new vectors, dense populations, changing conditions, or resistance to control measures.
• Climate change can shift vector ranges north and south as warmer conditions expand mosquito survival, breeding season, and transmission range. Rainfall changes and standing water also matter.
• Poverty and weak infrastructure increase spread when communities lack sanitary waste disposal, clean drinking water, sewage treatment, health care, and disease surveillance.
• Human environmental changes also matter:
  • travel moves infected hosts or vectors
  • urban crowding increases transmission
  • deforestation and settlement increase contact with wildlife reservoirs
  • poor waste management supports rodents and mosquito breeding

How Disease Cycles Are Broken

Control works best when it matches the transmission pathway.

• Source and reservoir controls
  • treat infected people
  • monitor animal reservoirs
  • reduce unsafe contact with infected animals, tissues, or fluids
• Pathway controls
  • sewage treatment
  • drinking water protection and disinfection
  • remove standing water
  • cover stored water
  • improve ventilation
  • clean contaminated surfaces
  • manage waste to reduce rodents and mosquitoes
• Host protections
  • mosquito nets and repellents
  • safe-sex practices
  • isolation during contagious illness
  • vaccination when available
  • testing and prompt treatment

A classic APES move is matching solution to disease. Cholera control means sanitation and clean water. Malaria, West Nile, and Zika control means mosquito control and bite prevention. Tuberculosis control means reducing respiratory exposure, crowding, and poor ventilation.