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Science Olympiad » Disease Detectives Event » Sample Problems and Answer Keys
Problem: E. coli and Salmonella

Read the following summary.

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Answer Key

An outbreak of E. coli O157:H7 gastroenteritis occurred in central Nebraska in February and March of 1999. E. coli O157:H7 is a bacteria that produces a toxin that can cause diarrhea (often bloody), abdominal cramps, and can affect the kidneys. E. coli O157:H7 can be transmitted person-to-person via the fecal-oral route. E. coli O157:H7 can also be transmitted by contaminated food or water. Nationwide, there has been evidence that ill food handlers may have been the source of an outbreak. Children aged less than 16 years and adults aged greater than 65 years are at greater risk of developing complications, including kidney failure. A total of 72 persons became ill. Nearly all of the affected person experienced diarrhea and abdominal cramps. Eight persons required hospitalization, including one child who required dialysis due to kidney failure. Of the 72 ill persons, 65 were exposed while dining at the family reunion on February 26 and seven were exposed secondarily.


  1. Construct 2 case definitions differentiating how the ill persons were exposed. (5 points each)

    Primary case-patient:


    Secondary case-patient:


Telephone interviews were conducted over a period of two weeks to obtain food and illness history. Analysis of the food history and illness data yielded two food items associated with illness. The following are the 2 x 2 tables for the food items.

Ranch Dressing


Got ill?

Yes No Total
Ate the dressing?      
Yes  18 17 35
No  46 54 100
Total  64 71 135

Iceburg Lettuce


Got ill?

Yes No Total
Ate the lettuce?      
Yes  57 29 86
No  8 40 48
Total  65 69 134
  1. Calculate the attack rate for each food item. (5 points each)

  3. Which food item explains the greater percentage of illness? (3 points)

  5. During the course of the environmental investigation, it was learned that the ranch dressing was used in preparation of the seafood salad. Describe how you would account for this in your analysis. (10 points)

  7. As well as interviewing the attendees, another important group to interview is the food handlers. These interviews will help shed light on the origin of the outbreak. Please list five items of information that you should obtain from the food handlers. (10 points)

  9. John Snow detected the source of an outbreak of (2 points)

    A. Typhoid fever
    B. Cholera
    C. Malaria
    D. Plague


  10. John Snow stopped the above outbreak by removing the handle of a neighborhood water pump. (2 points)



  11. A cohort study is used in an outbreak setting when a complete list of participants is available. (2 points)


  12. The measure of risk used in a cohort study is the odds ratio. (2 points)



  13. A case-control study is used in an outbreak setting when a complete list of participants is available. (2 points)



  14. The measure of risk used in a case-control study is the odds ratio. (2 points)


The following co-workers became ill following an in-office Chinese New Year celebration held on February 3 at 12:00 PM:

Chris (February 4, 4:00 a.m.)
Karen (February 4, 7:30 a.m.)
Bob (February 4, 10:00 a.m.)
Tom (February 4, 10:00 a.m.)
Josh (February 4, 8:00 p.m.)
Jane (February 4, 11:30 p.m.)
Adi (February 4, 12:00 p.m.)
Wayne (February 4, 11:00 a.m.)
Kim (February 4, 9:00 a.m.)
John (February 4, 4:12 p.m.).

  1. Please construct an epidemic curve using 2-hour increments on the graph paper provided. Please include a legend for the figure. (5 points)

  3. Describe the historical (1950 to 1996) trend of measles in the United States, using the attached graph. Include your interpretation of the impact of measles vaccine. (10 points)

  5. In the late 1980s and early 1990s, outbreaks of measles occurred on numerous college and high school campuses. Give your impression(s) as to what factors allowed this to occur. (5 points)

  7. As a result of this increase of measles, what public health recommendation(s) was/were made? (5 points)


  8. Describe the trend of salmonellosis in the United States from 1966 to 1996 and include three possible reasons for the change. (10 points)

  10. Salmonella is detected and grown during routine stool culture for enteric pathogens. Salmonella is further typed by identifying antigens on its cell body (O antigen) and flagella (H antigen) to determine setotypes. Serotypes are very useful epidemiologically. For example, S. enteriditis is the serotype most frequently associated with eggs. An increase in an uncommon serotyope raises suspicion of an outbreak. Using the graph on salmonella serotypes, describe the trend of salmonellosis in the United States from 1971 to 1996. How does this change your previous interpretation of salmonellosis in the United States? (10 points)

Answer Key

Tiebreaker #1

Please match the following outbreak settings with the agent causing the outbreak. Some diseases may be used more than once or not at all.

Outbreak Settings Agent Causing Outbreak
  1. __ Kitwit, Zaire, 1995
  2. __ Jack-in-the-Box, Pacific NW, 1993
  3. __ New Mexico/Arizona Four Corners, 1993
  4. __ Meatpacking plant employees, Omaha, 1999
  5. __ Public school students eating frozen strawberries, United States, 1997
  6. __ Restaurant-associated, Kearney, 1999
  7. __ Mosquito-associated, New York City, 1999
  8. __ "Party in the Pasture" or Cornstalk, Illinois, 1999
  9. __ Laboratory Monkeys, Reston, VA, 1989
  10. __ Salad-bar associated bioterrorism event, Oregon, 1987
  1. Shigella
  2. Hepatitis A
  3. Salmonella
  4. Ebola/Ebola related viruses
  5. E. coli O157:H7
  6. Rubella
  7. Hantavirus
  8. Measles
  9. West Nile Virus
  10. Botulism

Tie breaker #2

Please calculate the relative risk for iceberg lettuce using the 2x2 table in Question #2.

Answer Key

This page last reviewed April 24, 2007

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