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 of disease 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, from one person to another such as flu, common cold and COVID-19.


Contagious disease

A contagious disease is a subset category of transmissible diseases, which are 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 airborne route among other routes.


Transmissible disease

A disease that is transmitted from one individual to another by infection, through physical contact.



A contagious bacterial disease characterized by fever and delirium, typically with the formation of buboes (bubonic plague) and sometimes infection of the lungs (pneumonic plague).

Bubonic Plague

A serious, sometimes fatal, infection with the bacterial toxin Yersinia pestis, transmitted by fleas from infected rodents and characterized by high fever, weakness, and the formation of buboes, especially in the groin and armpits.

Pneumonic Plague

A form of plague characterized by lung involvement.

Septicaemic Plague

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, other animals, and other living hosts



Pasteurization or 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 with what we now know.

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.

Such stars of the immunology world include: John Snow and William Budd, Louis Pasteur, Robert Koch, Edward Jenner and Alexander Fleming.

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- protozoa and it was a type of bacteria that caused the silkworms to have diarrhoea.

Louis is credited with finalising the ‘Germ Theory’. The Germ theory states that disease is caused by micro-organisms. He also became famous by mastering the art of fermentation and the process of pasteurisation.”

Robert Koch

Robert Koch

Robert Koch came to fame by working on Anthrax 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 real claim to fame was developing a new method for discovering micro-organisms 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 in physiology or medicine. His claim to fame was discovering the antibiotic penicillin which is still widely used today.

From working with mould Alexander discovered that some of the mould just happened to kill some bacteria. It was an accidental discovery.

Alexander was knighted in 1944 for his work and know gets called Sir.

Barry Marshall

Barry Marshall

Barry Marshall has an unsual way of testing his scientific theories. Barry had a theory that stomach ulcers were caused by bacteria.

Barry decided to test his theory by growing the bacteria and then swallowing it himself. He then performed a medical test called an endsocopy to test for the disease and would you believe it- he had a stomach ulcer!

In 2005 Barry Marshall was awarded the Nobel Prize in Physiology and Medicine.

The plague

The plague in 1665 was caused by the bacterium Yersinia pestiswhich is spread by fleas on black rats. This disease killed mostly children as their immune system is under-developed.

It is a mutation of same bacterium that killed over 50 million people in Europe from 1348-1364.

A bacterium mutates because a bacterium 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, which lasted up to a week. There was some chance of surviving if the buboes burst.

The amount of the population who died, often referred to as the ‘mortality rate’, was 50%. 

Pneumonic Plague

A variant of the bubonic plague 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 of the bubonic plague was the septicaemic plague, which infected the blood. Again victims died quickly although the mortality rate was 100%.

Treatment 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 cats and humans with pneumonic plague.

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 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 often.
  • More people are inhabiting areas that were previously wildlife areas. Perhaps it our fault for invading 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: )

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’.