3 Viruses Go "Viral"

Activity 2

How Contagious Is it?


Have students read A Tale of Two Epidemics. In this reading students explore two epidemics—Severe Acute Respiratory Syndrome (SARS), spread by direct contact and the recent epidemic of Zika virus that is transmitted by a mosquito vector and implicated in microcephaly in newborns and in paralysis in adults. Students consider how the mode of transmission and other factors affect the emergence of an infectious disease and its escalation from a localized outbreak to an epidemic.

Students compare the two epidemics and use information about the biology and ecology of the viruses to propose ways the epidemics could be halted. At the end of the reading students revisit the idea that diseases that are transmitted by direct or indirect contact with infected individuals are considered contagious and are introduced to the mathematical construct, R0, that enables an estimation of the degree to which viruses or bacteria are contagious. Students read more about R0 in Reading 3 and work with it in the remainder of the module.



You may want to initiate a discussion with questions such as:

  • What characteristics do SARS and Zika epidemics have in common?

    They both most likely started out as small, localized outbreaks and were spread globally by travelers. Spread may have been accelerated by mutations in the virus.

  • How do the biology and ecology of Zika virus and SARS virus differ?

    SARS is spread by direct contact whereas a mosquito vector transmits Zika. Bats are the animal reservoir of the SARS virus whereas no animal reservoir of the Zika virus has been identified yet.

  • Using your understanding of the two viruses, how would you design an intervention for each?

    Isolating individuals who either had the disease or who had come in contact with infected individuals effectively halted the SARS epidemic. Possible ways to stop the Zika epidemic would involve eradication of the mosquito vector and clearing up trash near human habitation that serves as mosquito breeding grounds. An effective vaccine could stop both diseases.)

  • Using your understanding of the two viruses, explain whether these viruses and their diseases could ever be eradicated.

    It is unlikely that SARS could ever be eradicated since the involvement of an animal reservoir makes elimination of the virus almost impossible. Since a reservoir for Zika virus has not be identified, it is not known whether eradication is possible.



The purpose of the activity is to introduce students to the mathematical construct R0 that predicts the number of people who can contract an infectious disease from one infected individual (secondary infections), and, therefore, is a measure of contagiousness of a disease under certain conditions. Students learn about the uses of R0 and its limitations in predicting the course of an epidemic. They compare the R0 values of different viruses and consider the basis for the differences in infectivity.

Students then use a second mathematical equation to determine the percentage of a population that needs to receive the measles vaccine in order to prevent an epidemic. Students then read about the concept of community or herd immunity.

In a mock town meeting students take sides as to whether the Department of Public Health in their city should enforce a stricter vaccination policy that would require all children to be vaccinated against childhood infectious diseases. Students, either as a team for vaccination or against vaccinations prepare to present and discuss the role of vaccination in disease prevention, an analysis of the arguments for and against vaccination using evidence from the literature and the ethical issues surrounding vaccinations.


  1. Randomly assign students the role of being either for or against a stricter vaccination policy.
  2. Individually, have students read the introduction to the activity and sections about R0. Encourage students to watch the video that explains R0.
  3. Before having them address the task, conduct a discussion about the possible reasons the R0 values may differ from virus to virus, the value of knowing R0 of a virus, and its limitations.
  4. After the students have completed their calculations to determine the percentage of the population required to prevent a measles epidemic, have them view the interactive that demonstrates the difference in the spread of measles when the population is 80% vaccinated vs. 95% vaccinated and compare their findings to that of the interactive.
  5. Have students read about community immunity and discuss how the interactive map relates to that concept. Students should understand that the map demonstrates that when 95% of the population is vaccinated, most (but not all) of the 5% unvaccinated individuals are protected from infection. When only 80% are vaccinated protection through community immunity does not happen.
  6. Students should prepare notes on
    • the role of vaccination in preventing measles outbreaks;
    • their calculation of the percentage of the population that needs to be vaccinated to prevent a measles outbreak in the community, including an explanation of how this number was determined;
    • a discussion of how vaccinating a certain percentage of the population can prevent outbreaks and epidemics;
    • their analysis of the pros and cons of vaccination, including evidence for and against arguments on both sides;
    • their conclusions as to whether vaccination for all children should be required including justification of their conclusion using evidence from the literature.
    • a discussion of the possible of ethical implications of requiring vaccination and of not vaccinating a child.

Students then work in two teams, one to prepare a presentation in favor of a stricter vaccination policy and another against a stricter vaccination policy. Remind students that they will participate in a discussion after the presentations. A vote will be held at the end of the discussion. Remind them also that they should feel free to change their minds regardless of which team they were on.

Short readings are provided to initiate student thinking and writing. Students should be encouraged to seek further information about these topics using valid sources. Remind them that they need to cite their sources in their reports. You may want students to present the highlights of their reports in a discussion or in a poster.

Links to Background Readings: