Changing ideas about disease

Lesson 4

How infections have effected society and the changing ideas in society brought about by science

Learning outcomes & key terms

  • Students can define infection
  • Students will understand how ideas about infection and transmission have changed from medieval time to the present as knowledge has developed
  • Students will know the scientists who brought about the change of ideas, and their contribution, to our understanding of infectious disease

Infectious disease 

Infectious diseases are caused by pathogenic microorganisms, such as bacteria, viruses, parasites or fungi; the diseases can be spread, directly or indirectly


Contagious disease

A contagious disease is transmitted to other persons, either by physical contact with the person suffering the disease, or by casual contact with their secretions or objects touched by them or the airborne route



A contagious bacterial disease characterised by fever and delirium, typically with the formation of lumps called ‘buboes’ (bubonic plague) and sometimes infection of the lungs (pneumonic plague)

Bubonic plague is a serious, sometimes fatal, infection with the bacterium Yersinia pestis, transmitted by fleas from infected rodents and characterised by high fever, weakness, and the formation of buboes, especially in the groin and armpits

Pneumonic plague is a form of plague characterised by lung disease

Septicaemic plague is an especially dangerous form of plague in which the infecting organisms invade the bloodstream

Germ theory

The ‘germ theory’ of disease is the currently accepted scientific theory for many diseases. It states that microorganisms known as pathogens or “germs” can lead to disease

These small organisms, too small to see without magnification, invade humans, animals, and other living hosts eg plants



Pasteurisation is a process in which water and certain packaged and non-packaged foods are treated with mild heat, usually to less than 100 °C, to eliminate pathogens and extend shelf life of the food



A microorganism, or microbe, is a microscopic organism, which may exist in its single-celled form or in a colony of cells



Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye

Scientific understanding, including models and theories, are contestable and are refined over time through a process of review by the scientific community (ACSHE157)

  • considering how ideas about disease transmission have changed from medieval time to the present as knowledge has developed

Infectious diseases in history

An infectious disease is also known as a contagious disease or transmissible disease

Infection is the invasion of a host organism’s bodily tissues by disease-causing organisms, their multiplication, and the reaction of host tissues to these organisms and the toxins they produce

The idea of contagious disease first appeared during the Renaissance

By 1723, there was a large advance in the technology of microscopy allowing the visualisation of micro-organisms

In the past it was suggested that disease was caused by unseen creatures or bad smells. It’s hard to believe given what we know now

Even after microscopes were invented and micro-organisms were discovered, scientists still didn’t believe that they were the cause of the disease – it was like trying to convince the scientists that an ant could kill an elephant

Giants in the field of infectious diseases

There are some scientists who have really helped the world of infectious disease advance – without their wisdom we would not know what we do know

Stars of the immunology/infection world include: John Snow and William Budd, Louis Pasteur, Robert Koch, Edward Jenner, Alexander Fleming Howard Florey and Johanna Westerdijk

Louis Pasteur

Louis Pasteur

In the 1860s, Louis investigated a disease that attacked silkworms. He found that illness in the silkworms was caused by a pathogen – a protozoan that caused the silkworms to have diarrhoea

Louis is credited with finalising the ‘Germ Theory’ that disease is caused by micro-organisms. He also became famous by mastering the art of fermentation and the process of pasteurisation – he is also famous for the maxim that ‘fortune favours the prepared mind.’

Robert Koch

Robert Koch

Robert Koch came to fame by working on anthrax, cholera and tuberculosis. Koch found out that the anthrax microbe produced spores that lived for a long time after an animal had died. He also proved that these spores could then develop into the anthrax germ and could infect other animals

Possibly Koch’s best claim to fame was developing a new method for discovering the role of micro-organisms in disease, known as Koch’s Postulates

Koch’s Postulates:

  1. The organism must always be present, in every case of the disease
  2. The organism must be isolated from a host containing the disease and grown in pure culture
  3. Samples of the organism taken from pure culture must cause the same disease when inoculated into a healthy, susceptible animal in the laboratory
  4. The organism must be isolated from the inoculated animal and must be identified as the same original organism first isolated from the originally diseased host
Sir Alexander Fleming

Alexander Fleming

Sir Alexander Fleming received the Nobel prize – his claim to fame was discovering the antibiotic penicillin which is still widely used today

From his work with penicillium mould, Fleming discovered that the mould happened to kill some bacteria – it was an accidental discovery

Fleming, along with Howard Florey and Ernst Chain, were awarded the Nobel prize in 1944

Barry Marshall

Barry Marshall

Marshall had a theory that stomach ulcers were caused by bacteria – he had an unusual way of testing his scientific theories

Barry decided to test his theory by growing the bacteria and then swallowing it himself – he then had a medical test performed on himself called an endoscopy to look for the disease and – would you believe it – he had a stomach ulcer!

In 2005, Barry Marshall was awarded the Nobel Prize

The Plague

The plague is caused by the bacterium Yersinia pestis, which is spread by fleas on black rats – this disease mostly killed children as their immune system is under-developed

It killed over 50 million people in Europe from 1347-1364

A bacterium mutates because a bacterial colony has natural variation in their genes.  A change in environment may be more suited to one variation of genes than another – this variation will thrive and the other variations will decrease in number

Bubonic Plague

The bubonic plague was a painful disease, with black buboes, or swellings, in the groin and armpits. There seemed to be some chance of surviving if the buboes burst

The percentage of people who died, often referred to as the ‘mortality rate’, was 50%

Pneumonic Plague

A variant was the pneumonic plague, which attacked the lungs. Victims died quickly, in one or two days. The mortality rate in this case was 90%

Septicaemic Plague

Another variant was the septicaemic plague, which infected the blood – again victims died quickly although the mortality rate was close to 100%

Management of the Plague

Doctors thought that the plague was caused by “bad air” and wore masks filled with aromatic herbs to prevent catching the disease

As we now know, people most commonly acquire plague when they are bitten by a flea that is infected with the plague bacteria – people can also become infected from direct contact with infected tissues or fluids while handling an animal that is sick with or that has died from plague Finally, people can become infected from inhaling respiratory droplets after close contact with infected humans and animals

Why are diseases different today than in the past?

There is no quick answer to this question – many scientists agree that most of the new diseases were first spread by animals

Other theories include:

  • The climate is getting warmer, making it easier for insects that spread the disease
  • People are travelling more and as they do they are taking their germs with them
  • Most people now live close together in big cities where they are in contact with more people
  • More people are inhabiting areas that were previously wildlife areas – perhaps it our fault for encroaching on these areas?

Advanced explanation of immune system; Your immune System: Natural Born Killer. Crash Course Biology #32

More basic explanation: animation of how the immune system works (direct link: )

See Ted Ed video on Rosalind Franklin: DNA’s unsung hero – during Emmanuelle Charpentier’s studies of Streptococcus pyogenes, one of the bacteria that cause the most harm to humanity, she discovered a previously unknown molecule, tracrRNA. Her work showed that tracrRNA is part of bacteria’s ancient immune system, CRISPR/Cas, that disarms viruses by cleaving their DNA.

Short videos including immune immune system structure and function – overview, non-specific and specific response. Unit – Barriers to Pathogens.

If pathogens get through the body’s first two lines of defense, a third line of defense takes over. This third line of defense involves the immune system. For a cartoon introduction to the immune system, watch this video: Unit plan on acting out the immune response. Great ideas for extension. Unit plan studying the body’s defense mechanisms YouTube video: Your Immune System: Natural Born Killer by Crash Course Biology (advanced)

Analogies of Immune System: Robbing a bank Protecting a castle Infection, detection, protection. Series of activities and games focusing on the immune system. These are geared at younger learners but provide good ideas for adapted technology creation. more aligned with lesson 2, this is a great idea for an interactive game though.

Which type of barrier to pathogens is in your opinion the most effective? Explain why.

Class extension activity: In the class activity “Acting Out the Immune Response” at the URL below, students will act out a nonspecific and then a specific response to an invading pathogen. Each student will have an assigned role in the activity, and props will be used.


Society has battled infectious diseases since the beginning of recorded history.  Leading scientists have broken new ground and understood diseases by building a germ theory and then applying good science

Major pandemics, such as the plague and coronavirus or influenza, have been battled with ‘the prepared mind’ (look up what Pasteur said)



1) Pneumonic plague is a variant that
a) Attacked the lungs
b) Caused painful swelling in the groin and armpits
c) Infected the blood
d) Caused 50% of people who caught it to die


2) The scientist who won the Nobel Prize by testing his theory on himself by giving himself a stomach ulcer was
a) Louis Pasteur
b) Robert Koch
c) Barry Marshall
d) Alexander Flemming


3) The scientist who received the Nobel Prize for discovering penicillin was
a) Louis Pasteur
b) Robert Koch
c) Barry Marshall
d) Alexander Flemming


4) Koch’s postulates include
a) The organism must always be present in every case of the disease
b) The organism must be isolated from a host containing the disease and grown in pure culture
c) Samples of the organism taken from pure culture must cause the same disease when given to a healthy susceptible animal
d) All of the above