Outline
OVERVIEW
22.1
Rationale: This lesson reinforces the concept that infectious diseases are
caused by pathogens, and that although identifying the pathogen may be
difficult it is important for treatment and/or prevention. The lesson also emphasizes that being able to identify infectious pathogens is a relatively new
skill, largely because identification usually requires special tools, for example
microscopes.
Do Now: Defining a disease as infectious has a number of implications:
people with the disease have been infected with the pathogen; the pathogen
is causing the disease; and the infection can be spread. Students design
tests to determing whether a disease is infectious, thereby empirically arriving
at Koch’s postulates— important criteria for identifying infectious agents.
Typhoid Mary Video: This video illustrates the inherent difficulties in identifying the cause of an outbreak of an infectious disease. The techniques used
to identify the infectious disease are critical. In the discussion, students are
exposed to past challenges encountered in identifying infectious agents to
current problems.
Wrap Up: In preparation for the next lesson, introduce the idea that to prove
that a disease is infectious you need to be able to identify the pathogen.
Homework: Students will consolidate their understanding of the criteria
needed to establish whether a disease is infectious by reflection on how
cholera was identified in Victorian London.
The
Lesson
Plan
Infectious Disease DetectivesTyphoid Mary
1. Do Now (5 min):
Introduce the theme of Unit 2, What does it mean to have an infectious disease? Using the definition of ‘infectious disease’ that the
class helped to generate, answer the questions ‘How do you know
that a disease is infectious?’ and ‘Do you think cancer is an infectious
disease?’ Does being exposed to an infectious agent constitute proof
that the agent has caused your disease?
2. Activity (30-35 min):
Typhoid Mary Video. Discuss the film (5-10 min), drawing on examples
from the video that show how Mary was found to have caused the
disease.
3. Wrap Up (5 min):
In preparation for the next lesson, introduce the idea that to prove that
a disease is infectious you need to be able to identify the pathogen.
LE S S O N
Le s s o n
Unit1.2
53
1.
NOW
DO
This lesson re-emphasizes a concept introduced in Unit 1: most infectious agents can’t be seen with the naked eye.
Thus it is often difficult to prove that a particular infectious agent causes a
disease.
What do you see?
■■ Our bodies are covered in commensal microbes.
Are you sick?
How then do you distinguish between microbes and pathogens?
If you can’t find a microbe is that proof that the disease is
not infectious?
If possible, gather the students round a microscope and have a few slides ready.
Ask the students:
How do you know if a disease in infectious? Is cancer caused by
an infectious disease?
■■ Guide the students to articulate that because infectious agents are so
small special tools will be needed to identify them. For example, you
may be able to see bacteria under the microscope.
LE S S O N
2.1
Is finding a microbe proof that it causes the disease?
■■ Ask a few students to take a swab from the inside of their mouths onto
the slide and give the students a chance to look at them under the
microscope.
■■ In fact, scientists are constantly discovering that diseases once
thought of as non-infectious are in fact caused by infectious
agents. For example, many cancers are now associated with
infectious agents (one example is cervical cancer – you can also
mention that a vaccine is now available to protect against cervical
cancer, and that young people (both boys and girls) of their age
can be vaccinated.
■■ Students may also say that an infection could be identified as
it travels within the human population, for example the people
with influenza in the Spanish flu epidemic were exposed to other
people with influenza. This leads to the concept of using correlation between exposure and disease to identify whether a disease
is infectious.
What if an infectious diseases only causes symptoms in
a small number of infected people, making the correlation
look weak?
■■ This point will be the focus of the video in this lesson.
54
2.
Activity
Typhoid Mary Video
Show the video from 4:30 – 31:00 minutes. The selected video
clip is approximately 25 minutes.
Ask the students:
Why was it so hard to identify Mary as the source of the
disease?
■■ Mary was apparently perfectly healthy, but she was known to be
responsible for a typhoid “epidemic”. She was the first person
known who could transmit disease without apparently being
infected themselves.
Would it be hard to identify the source of a disease if only
50% of people infected show symptoms?
■■ Yes, because it would be hard to identify an appropriate correlation.
Discussion of the Film
Important Points:
■■ Mary Mallon (commonly known as “Typhoid Mary”) is the most famous
asymptomatic carrier of an infectious disease.
Is there a way that Mary’s righteous indignation may have
been justified?
■■ Actually it is possible that she was born with the disease, as
her mother had typhoid fever during her pregnancy. This would
account for why she always vehemently denied ever having had
typhoid herself.
■■ She was a young cook at the beginning of the 20th century who was
responsible for infecting at least 53 people with typhoid, three of whom
died from the disease.
LE S S O N
2.1
■■ She strenuously denied that she was infected with typhoid and that she
played a role in spreading the disease, and refused to stop working as
a cook.
■■ As a consequence she was forcibly quarantined and even died in
quarantine.
55
3.
Wrap Up
■■ As far back as the first century BC the Roman Marcus Terentius
Varro wrote a book called ‘On Agriculture’ in which he warns
against locating a homestead near swamps , probably to diminish
the contraction of malaria.
■■ The famous Islamic physician Abu Ali ibn Sina (also known by his
Roman name of Avicenna), in his book called ‘The Canon of Medicine’ written in 1020, stated that ‘bodily secretion is contaminated
by foul foreign earthly bodies before being infected’. He also
hypothesized that tuberculosis might be an infectious disease. He
successfully used quarantine to limit its spread.
Important points:
The image in this slide shows a text from the 1st century AD that describes
infectious agents as creatures that cannot be seen by the eyes.
People are smart. They’ve known they can catch diseases from water,
food and other people for a long time, but they couldn’t prove that the
diseases were caused by an infectious agent until they could positively
identify disease-causing microbes!
Tell the students to think about how people
knew of these small creatures if they could not
see them.
LE S S O N
2.1
■■ The point here is correlation and observation. In the quote shown in
the slide there is discussion of the small creatures floating in the air and
entering the body via the mouth and nose. So exposure to air around
sick people must have been correlated with contraction of the disease.
Below are more examples of correlation that lead to the hypothesis of
microbes that cause disease.
56