many infectious diseases. Because immunized people do not develop
diseases that must be treated with antimicrobial drugs, opportunities for
pathogens to evolve and disseminate drug resistance genes are reduced.
Thus, mass immunization reduces the need to develop newer and more
expensive drugs.
As long as a disease remains endemic anywhere, vaccination programs
must be maintained everywhere. This is because an infected person can
travel anywhere in the world within 24 hours. Once global vaccina tion
programs eliminate the infectious agent (as in the case of the smallpox
virus), vaccination is no longer necessary, and the expense of those
programs is also eliminated. It is estimated that the United States has
saved $17 billion so far as a result of the eradication of smallpox (which
cost, according to the World Health Organization, $313 million across a
10-year period).
Lapses in vaccination programs explain the re-emergence of some
infectious diseases. For example, the diphtheria outbreak in Russia in
the early 1990s may have been due to lapses in vaccination programs
associated with the breakup of the Soviet Union. Inadequate vaccines
and failure to obtain required “booster shots” also explain some disease
re-emergence. The dramatic increase in measles cases in the United States
during 1989–1991 was likely caused by failure to give a second dose of
the vaccine to school-age children. The American Academy of Pediatrics
now recommends that all children receive a second dose of the measles
vaccine at either ages 4–6 or 11–12 years.
Seasonal vaccination for influenza is one of the most underused preventive
measures in the United States. Morbidity and mortality might be mitigated
if there were better compliance with vaccination recommendations.
This lesson and Lesson 3, Superbugs: An Evolving Concern, both provide
explanations for the re-emergence of some infectious diseases. Lesson 3
explained that some re-emerging diseases are due to the evolution of
antibiotic resistance among pathogens. Lesson 4, Protecting the Herd,
introduces students to the idea that the re-emergence of other infectious
diseases can be explained by a failure to immunize a sufficient proportion
of the population. On the first day of the lesson, students learn that
epidemics can be prevented by immunizing part of the population, leading
to herd immunity. The concept of herd immunity is elaborated in the
optional, second day of the lesson. Here, students learn that the threshold
level of immunity required to establish herd immunity (and thus prevent
epidemics) varies depending on the transmissibility of the disease, the
length of the infectious period, the population density, and other factors.
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For Day 1
In Advance
Photocopies and Transparencies
Equipment and Materials
• 1 copy per student of
• 1 overhead projector
Masters 4.1, 4.2,
• red, pink, and black cards
• 2 copies per student of
(1 of each per student)
Master 4.3
• folded pieces of paper labeled
• 2 transparencies of Master 4.3
“immune” and “susceptible”
(make enough of each for
half the students)
(Optional) For Day 2
Photocopies and Transparencies
Equipment and Materials
• 1 copy per student of
• 1 overhead projector
Master 4.4
• blank transparencies
• 1 transparency of Masters 4.5
• (Optional) Computers with
and 4.6
access to the Internet
Note to teachers: If you do not have enough computers with Internet access,
you will not be able to conduct the optional Day 2 of this lesson.
DAY 1
Procedure
1. Introduce the lesson by distributing one copy of Master 4.1, Measles
Outbreak at Western High, to each student and asking the students
to read it.
The scenario described on Measles Outbreak is fictitious, but it’s based
on an outbreak of measles that occurred in Washington State in 1996.
An alternate way to introduce the lesson is to assign students to make a
list of the childhood diseases that they, their parents (or someone from
their parents’ generation), and their grandparents (or someone from
their grandparents’ generation) had. Explain that “childhood diseases”
means diseases that people usually have just once and do not get again
(for example, chicken pox). Explain that you do not mean diseases
like the flu, strep throat, and colds. On the day you wish to begin the
lesson, ask students to name some of these diseases, then ask them to
count the number of different diseases each generation in their family
had. Total these numbers across all of the students in the class and ask
students to suggest why (in general) their parents and grandparents had
more diseases than they did. Students likely will suggest (correctly) that
vaccination against many diseases is now available.
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Student Lesson 4
Emerging and Re-emerging Infectious Diseases
2. After students have read Master 4.1 , ask them to speculate about
what might have happened to cause a sudden outbreak of a
disease such as measles that normally, today, is relatively rare in
the United States.
This is an opportunity to
point out that research in
Students will likely know that most children in the United States today
are vaccinated against measles. They may speculate that the students at
microbiology and related
Western High were not vaccinated, or that the vaccine did not work in
disciplines in the past 50
their cases, or even that the pathogen causing this form of measles was
years has led to the
somehow able to evade the immune defenses that had been triggered
development of many
by the vaccinations these children received.
vaccines in addition to the
measles vaccine. Children
3. Distribute one copy of Master 4.2, A Little Sleuthing, to each student, of the 1990s who receive
and ask the students to read the story and think about the question
that ends it.
recommended
vaccinations are protected 4. Point out that despite the success of the measles vaccine, there from many infectious
continue to be small outbreaks of measles in the United States.
diseases that plagued
Explain that the key to understanding why this is true and to
children in the past,
answering the question that ends the story about Western High
including diphtheria,
lies in understanding how disease spreads in a population.
whooping cough,
5. Explain to students that to help them understand how disease spreads
measles, hepatitis B,
in a population, they will participate in a simulation of the spread of a
and chicken pox.
fictitious disease you will call the “two-day disease.” Give two copies
of Master 4.3, Following an Epidemic, to each student and display
a transparency of this master. Then, direct students to perform
two simulations of the spread of two-day disease, according to the
instructions provided on pages 95–97, immediately after the lesson.
An “epidemic” is typically defined as “more cases of a disease than
is expected for that disease.” Although this is not a very specific
definition, it does make clear that whether scientists call an outbreak of
a disease an epidemic depends on the specific disease involved. Though
there is no distinct line between an “outbreak” and an “epidemic,”
epidemics are generally considered to be larger in scale and longer
lasting than outbreaks. Today, five cases of measles within a population
could be considered an epidemic because no cases are expected.
For this simulation, assume that an epidemic is in progress if 25 percent
or more of the population is sick at one time.
Observations that students might make about the table and graph that
result from the first simulation include
• an epidemic occurred because a large portion of the class was sick
at the same time;
• at the beginning of the epidemic, only a few people were sick in the
same day; in the middle of the epidemic, a lot of people were sick at
the same time; and at the end, only a few people were sick;
88
• by the end of the simulation, everyone was immune; and
• once it started, the disease spread rapidly.
Observations that students might make about the table and graph
that result from the second simulation include
• only a few people were sick on any one day;
• no epidemic occurred;
• at the end of the simulation, some people were still susceptible; and
• some people in the population never got sick.
Tip from the field test: Do a practice run of several days of the
simulation before you do the runs in which you collect data. This
will allow you to address any confusion students have about the
simulation and will make subsequent runs go much faster. If you
have time, you may want to repeat the simulation, especially the
second one, in which half the class is immune. In order for students
to observe herd immunity, some susceptible students in the population
should not get sick. Depending on the arrangement of immune and
susceptible students in the class (which is random), this may not
happen the first time you run this simulation.
6. Debrief the activity by asking, “Why did an epidemic occur in the first
population but not in the second?” and “Why didn’t all the susceptible
people in the second population get sick?” Introduce the term “herd
immunity” and describe it as a phenomenon that occurs when most
of the people in a population are immune to an infectious disease.
Susceptible people in the population are protected from that disease
because the infectious agent cannot be transmitted effectively.
Allow students to discuss their responses to the two questions before
you introduce the term herd immunity. Students will likely make
comments such as, “Everyone sitting near John was immune, so
the disease just died out.” At that point, you can respond by saying,
“Yes, what you have just explained is what epidemiologists call herd
immunity.” Then you can provide a more complete definition.
7. Ask students to explain, based on their experience in the disease-
transmission simulation, what would happen if measles vaccinations
dropped to a low level in a population.
Students should be able to explain that there would be many susceptible This step takes students to people in the population, so the disease would be transmitted from one
the major concept of the
to another without dying out. A measles outbreak or epidemic would
activity: The re-emer gence
occur. If students do not mention “re-emergence,” emphasize this point
of some diseases can be
by saying, “Yes, measles would re-emerge in the population.”
explained by immunity
levels that are below
the level required for
herd immunity.
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Student Lesson 4
Emerging and Re-emerging Infectious Diseases
8. Remind students about the measles-outbreak story. Ask them to
write a final paragraph to the story in which they use the term
herd immunity to answer the following questions:
Collect and review
• Why didn’t the unvaccinated or inadequately vaccinated students
students’ paragraphs to
and teacher at Western High get measles when they were children
rather than as teenagers or adults?
assess their understanding
of the major concept of the
Students should be able to explain that the unvaccinated or
activity. Address common
inadequately vaccinated students at Western High were protected
misunderstandings in the
by herd immunity when they were younger: Because most of the
next class session and
people around them were immune, the infectious agent could not
read two or three of
be transmitted from those people.
the best paragraphs
• Why is vaccination not only a personal health issue, but also a
to the class.
public health issue?
Vaccination is a public health issue because maintaining high levels
of immunity in a population prevents epidemics and protects the
small percentage of susceptible people from the disease.
DAY 2 (Optional) For classes with access to
the Internet
1. Open the activity by reminding students about two-day
disease and the simulation they completed. Then, ask them what
characteristics may vary between two-day disease and other diseases.
Point out that differences in these characteristics affect the likelihood
that an epidemic of a particular disease will occur and the percentage
of the population that must be immune to that disease to achieve
herd immunity.
Expect students to suggest that people who are sick may contact more
than one person per day, may be sick (and infectious) for more than
two days, may die from the disease, and may not get sick from just
one contact. Students may also point out that the disease may require
“intimate” rather than casual contact, or it may not require person-to-
person contact.
2. Ask students to predict what the results of the simulation would be
if they varied each of four characteristics of the disease: virulence
(the likelihood of dying from the disease), duration of infection, rate
of transmission (how contagious the disease is), and level of immunity
in the population. Insist that students provide some rationale for their
predictions. Write their predictions on the board or a blank transparency.
To help students think about this, you may wish to ask questions
such as, “Do you think there would have been an epidemic of two-day
disease if people sometimes died from the disease? If so, do you think
it would have been a more or less severe epidemic?”
90
Virulence, duration of infection, rate of transmission, and level of immunity
are the four parameters that the computer simulation will allow students to
vary. Students may make predictions such as, “The more virulent a disease
is, the greater the likelihood of an epidemic,” or “The higher the immunity
level of a population, the less likely it is that an epidemic will occur.”
3. Tell students they will use a computer simulation to investigate the
likelihood of an epidemic when they vary one of the four characteristics
they just discussed. Give one copy of Master 4.4, Disease-Transmission-
Simulation Record, to each student and ask students to work in their
groups. Assign each group one of the four characteristics to investigate
and direct students to circle this characteristic on the master.
Tell students that because the computer simulation uses a larger
population size, an epidemic is defined as an outbreak of disease
in which 10 percent or more of the population is sick at one time.
4. Tell students to go to this part of the Web site and click on “Protecting
the Herd”: http://science.education.nih.gov/supplements/diseases/
activities/. Explain briefly how to use the simulation, then direct students to use it to test their assigned characteristic. Explain
that groups should test four different levels of their assigned
characteristic and that they have 15 minutes to complete this
work before reporting their findings to the class.
You may wish to explain the following features of the simulation:
• Users can set each disease characteristic at a variety of levels
(as indicated on the screen).
• Users can have the simulation run automatically for 30 days or step
through those days one by one, depending on the button they click.
• To repeat a run or to change the settings and do another run,
users must click the Reset button.
• Once a run begins, users cannot change the settings unless they
click the Reset button.
You may want to suggest that the groups that were assigned the
virulence characteristic select four levels from the low end of the
available range (less than 0.1 or 0.2) to test. Because of the levels
students will be using for duration of infection and rate of transmission,
any disease that has moderate to high virulence rapidly dies out in a
population. Students will have more interesting results if they use the
lower levels for virulence.
A range from 0.001 to 0.1 encompasses estimated rates of transmission
for many infectious diseases. The algorithm for this simulation assumes
that each infected person makes 100 contacts per day. Thus, the range
of settings available to students is 0.1 (0.001 × 100) to 10 (0.1 × 100).
The simulation would have to be adjusted for populations that are more
or less dense than the one assumed by the simulation.
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Student Lesson 4
Emerging and Re-emerging Infectious Diseases
5. Reconvene the class and ask questions such as, “Did your predictions
match what you discovered using the simulation?” or “Were you
surprised by the results of the simulation?” Ask one of the groups
that investigated the effect of varying virulence level to read its
summary statement to the class. Invite other groups that investigated
that characteristic to add more information to the statement or to
disagree with it. Repeat this process for the other three characteristics
the groups investigated.
Students should have discovered the following, according to the
computer simulation:
Virulence. A disease that is not very virulent remains at a low level in
the population, whereas diseases that are quite virulent rapidly die out.
Real disease examples that show this are colds and Ebola hemorrhagic
fever. Colds are not very virulent, and infected individuals remain
contagious for several days. Thus, colds tend to remain at a fairly
constant low level in the population. Ebola fever is very virulent (50 to
90 percent mortality) and death occurs shortly after infection, lessening
the opportunities for an infected individual to spread the virus beyond
his or her immediate surroundings. Therefore, at least until recent
improvements in travel in areas where Ebola has occurred, it tended
to occur in isolated outbreaks that died out fairly quickly.
Duration of infection. As the duration of infection increases, infected
individuals have more opportunities to transmit the infection to others.
In turn, each secondarily infected individual has more opportunity to
infect still others. Therefore, because larger numbers of people become
infected within a short period of time, epidemics become apparent
sooner after introduction of infected individuals into the population,
reach a higher peak incidence, and last longer. Real disease examples
showing this are influenza and chicken pox.
Rate of transmission. According to the computer simulation, a disease
dies out at low levels of transmission, whereas it stabilizes and becomes
endemic at high levels. Real disease examples of this include malaria
and many diarrheal diseases. Public health measures and access to
medical care result in dramatically decreased transmission of these
diseases in the United States, but they remain endemic in developing
countries where such public health measures and medical care are
not readily available.
Initial percent immune. With virulence, duration of infection,
and rate of transmission set at the values for two-day disease, the
computer simulation predicts that an epidemic will not occur when
the proportion of immune people in the population is greater than
15 percent.
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6. Explain to students that computer simulations such as the one they
have explored are useful tools for epidemiologists, who use them to
make predictions about the likelihood that an epidemic will occur
in a particular population or to estimate the level of vaccination
coverage they must achieve to prevent epidemics in the population.
7. Challenge groups to use the simulation to estimate the level of
immunization required to prevent epidemics of three real diseases:
smallpox, polio, and measles. Assign each group one of the diseases
and display the transparency of Master 4.5, Characteristics of
Smallpox, Polio, and Measles, which provides the settings they need
for the simulation. Tell groups they have 10 minutes to complete
their work.
Smallpox was declared eradicated from the world in 1980. Because
epidemiologists knew it would not be possible to vaccinate
everyone in the world, they used mathematical models of the
spread of disease to estimate the level of vaccination coverage they
needed to achieve and maintain to establish herd immunity in a
population. (The computer simulation in this activity is based on
a similar mathematical model.) Epidemiologists knew smallpox
would eventually be eliminated because there would not be enough
susceptible people to transmit the sma