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3.6.1Additional lessons

Pathogens — IB Biology SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 70 of 142

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Part 1 of 5Types of Pathogen

3.6.1 Pathogens

A pathogen is an organism or virus that causes disease.

Bacteria
  • Prokaryotic cells that cause diseases such as cholera and tuberculosis, although most bacteria are harmless or useful.
Fungi
  • Some fungi infect skin or other tissues, for example the fungus that causes athlete's foot.
Protists
  • Single-celled eukaryotes such as Plasmodium, the protist that causes malaria.
Viruses
  • Non-cellular particles that reproduce only inside host cells, using the host's cell machinery to make more viruses.
Archaea
  • No archaea are known to cause disease in humans.

Part 2 of 5Infectious and Non-infectious Disease

Infectious (communicable)
  • These diseases are caused by pathogens and can be passed from one host to another, like cholera, malaria and tuberculosis.
Non-communicable
  • These diseases are not caused by a transmissible agent, and examples include cancer, cardiovascular disease and malnutrition.
Signs and symptoms
  • Each disease has a recognisable set of signs and symptoms, which helps with diagnosis and with finding the cause.
Aim of control
  • The best outcome is eradication, but control usually focuses on stopping transmission and treating the infected.
FeatureInfectiousNon-infectious
CausePathogenGenes, diet, environment or lifestyle
Spread between peopleYesNo
ExampleCholeraCardiovascular disease

Part 3 of 5Snow and Cholera

1
Cholera is caused by the bacterium Vibrio cholerae, which spreads through contaminated drinking water and food.
2
Infected people pass large numbers of bacteria in their faeces, and these contaminate water supplies if sanitation is poor.
3
In 1854 John Snow mapped cholera deaths in Soho in London and found they clustered around one water pump.
4
The pump handle was removed, the outbreak ended, and the case supported the idea that disease can be carried in water.
5
This led to the provision of clean water and safe sewage disposal, which are still the main defences against cholera.

Part 4 of 5Semmelweis and Childbed Fever

Both examples are used to show how careful observation leads to progress.

1
Childbed fever is a bacterial infection of the reproductive tract that killed many women after childbirth in the nineteenth century.
2
Ignaz Semmelweis noticed that deaths were much higher in the ward staffed by doctors who came from dissections than in the ward staffed by midwives.
3
He introduced hand washing with a disinfectant before examining patients, and the number of deaths fell sharply.
4
His work showed that careful observation and comparison can identify a cause before the pathogen itself is known.

Part 5 of 5Exam Checklist & Tips

3.6.1 – Key Points for the Exam

  • Pathogen An organism or virus that causes disease.
  • Infectious disease A disease caused by a pathogen that can spread between hosts.
  • Non-communicable disease A disease not caused by a transmissible pathogen.
  • Vibrio cholerae The bacterium that causes cholera.

Common Mistakes

  • Writing that all bacteria are pathogens; most are harmless and many are useful.
  • Stating that viruses are cells; they are non-cellular.
  • Calling cancer an infectious disease because it is serious.
  • Saying Snow discovered the cholera bacterium; he showed water was the route of spread.

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