Chapters 2-4
1. Which of the following is characteristic of a single-exposure, common-vehicle
outbreak?
a. Long latency period before many illnesses develop
b. There is an exponential increase in secondary cases following initial exposures
c. Cases include only those who have been exposed to sick persons
d. The epidemic curve has a normal distribution when plotted against the logarithm of
time
e. Wide range in incubation times for sick individuals
Rationale: Single-exposure, common-vehicle outbreaks involve a sudden, rapid
increase in cases of disease that are limited to persons who share a common
exposure. Additionally, few secondary cases develop among persons exposed to
primary cases. A histogram of the outbreak can plot the number of cases by
time of disease onset. In single-exposure, common-vehicle outbreaks, a log
transformation of the time of disease onset will often take on the characteristic
shape of a normal distribution (i.e., a bell curve) with the median incubation
time found at the peak of the curve.
2. Which of the following is a condition which may occur during the incubation
period?
a. Onset of clinical illness
b. Receipt of infection
c. Signs & symptoms of disease
d. Transmission of infection
e. Isolation of disease carrier through quarantine
Rationale: The incubation period is defined as the interval from receipt of
infection to the time of onset of clinical illness. Accordingly, individuals may
transmit infectious agents during the incubation period as they show no signs of
disease that would enable the isolation of sick individuals by quarantine.
3. Chicken pox is a highly communicable disease. It may be transmitted by direct
contact with a person infected with the varicella-zoster virus (VZV). The typical
incubation time is between 10 to 20 days. A boy started school 2 weeks after
showing symptoms of chicken pox including mild fever, skin rash, & fluid-filled
blisters. One month after the boy returned to school, none of his classmates had
been infected by VZV. The main reason was:
a. Herd immunity
b. All had been immunized prior to the school year
c. Contact was after infectious period
d. Subclinical infections were not yet detected
e. Disease was endemic in the class
Rationale: The disease is spread by contact with an infected individual who can
transmit the agent (VZV) to immunologically naive persons during the
,incubation period & for several days after onset of clinical illness. Since the boy
started school 14 days after showing signs consistent with chicken pox, it is
most likely that he was no longer infectious.
4. What is the diarrhea attack rate in persons who ate both ice cream & pizza?
a. 39/52
b. 21/70
c. 39/67
d. 51/67
e. None of the above
Rationale:
The attack rate in this example is defined as the number of persons who
develop diarrhea divided by the total number of people at risk. In this example,
the at-risk group is those who have eaten both ice cream & pizza. Of these 52
persons, 39 developed diarrhea.
5. What is the overall attack rate in persons who did not eat ice cream?
a. 30%
b. 33%
c. 35%
d. 44%
e. 58%
Rationale:
The attack rate is the number of persons with diarrhea (14 + 9) divided by the
total number of persons who did not eat ice cream (40 + 30).
6. Which of the food items (or combination of items) is most likely to be the
infective item(s)?
a. Pizza only
b. Ice cream only
c. Neither pizza or ice cream
d. Both pizza & icecream
e. Cannot be assumed from the data shown
Among persons eating ice cream, over 70% developed diarrhea regardless of
their pizza consumption (39/52 & 11/15). Among both groups of persons who did
not eat ice cream, each attack rate was equal to or less than 35% (14/40 &
9/30).
, 7. Which of the following reasons can explain why a person who did not consume
the infective food item got sick?
a. They were directly exposed to persons who did eat the infective food item
b. Diarrhea is a general symptom consistent with a number of illnesses
c. There may have been an inaccurate recall of which foods were eaten
d. All of the above
e. None of the above
Rationale: Without knowledge as to the specific agent in this instance, it is also
likely that it can be spread by direct contact with infected persons. Since
diarrhea is a general disease symptom, it is possible that several infectious
agents may be present at this meal or others eaten during the same time
period. Further, information regarding food consumption may have been
collected long after the disease episode. This may have led persons to
incorrectly remember the foods that they consumed.
An outbreak of gastroenteritis occurred at a boarding school with a student enrollment
of 846. Fifty-seven students reported symptoms including vomiting, diarrhea, nausea, &
low-grade fever between 10 p.m. on September 24 & 8 p.m. on September 25. The ill
students lived in dormitories that housed 723 of the students. The table below provides
information on the number of students per type of residence & the number reporting
illnesses consistent with the described symptoms & onset time. Calculate the attack
rate among all students at the boarding school.
1. Calculate the attack rate among all students at the boarding school.
57/846 = The answer is found by dividing the total number of cases (57) by the
total number of students (846). This equals 6.7%.
10. Calculate the attack rates for boys & girls separately.
a. For boys, the attack rate includes all cases (40 + 3) divided by the total number of students who are
boys (380 + 46). The attack rate is 10.1%.
b. For girls, the attack rate includes all cases (12 + 2) divided by the total number of students who are
girls (343 + 77). The attack rate is 3.3%.
11. What is the proportion of total cases occurring in boys?
1. Which of the following is characteristic of a single-exposure, common-vehicle
outbreak?
a. Long latency period before many illnesses develop
b. There is an exponential increase in secondary cases following initial exposures
c. Cases include only those who have been exposed to sick persons
d. The epidemic curve has a normal distribution when plotted against the logarithm of
time
e. Wide range in incubation times for sick individuals
Rationale: Single-exposure, common-vehicle outbreaks involve a sudden, rapid
increase in cases of disease that are limited to persons who share a common
exposure. Additionally, few secondary cases develop among persons exposed to
primary cases. A histogram of the outbreak can plot the number of cases by
time of disease onset. In single-exposure, common-vehicle outbreaks, a log
transformation of the time of disease onset will often take on the characteristic
shape of a normal distribution (i.e., a bell curve) with the median incubation
time found at the peak of the curve.
2. Which of the following is a condition which may occur during the incubation
period?
a. Onset of clinical illness
b. Receipt of infection
c. Signs & symptoms of disease
d. Transmission of infection
e. Isolation of disease carrier through quarantine
Rationale: The incubation period is defined as the interval from receipt of
infection to the time of onset of clinical illness. Accordingly, individuals may
transmit infectious agents during the incubation period as they show no signs of
disease that would enable the isolation of sick individuals by quarantine.
3. Chicken pox is a highly communicable disease. It may be transmitted by direct
contact with a person infected with the varicella-zoster virus (VZV). The typical
incubation time is between 10 to 20 days. A boy started school 2 weeks after
showing symptoms of chicken pox including mild fever, skin rash, & fluid-filled
blisters. One month after the boy returned to school, none of his classmates had
been infected by VZV. The main reason was:
a. Herd immunity
b. All had been immunized prior to the school year
c. Contact was after infectious period
d. Subclinical infections were not yet detected
e. Disease was endemic in the class
Rationale: The disease is spread by contact with an infected individual who can
transmit the agent (VZV) to immunologically naive persons during the
,incubation period & for several days after onset of clinical illness. Since the boy
started school 14 days after showing signs consistent with chicken pox, it is
most likely that he was no longer infectious.
4. What is the diarrhea attack rate in persons who ate both ice cream & pizza?
a. 39/52
b. 21/70
c. 39/67
d. 51/67
e. None of the above
Rationale:
The attack rate in this example is defined as the number of persons who
develop diarrhea divided by the total number of people at risk. In this example,
the at-risk group is those who have eaten both ice cream & pizza. Of these 52
persons, 39 developed diarrhea.
5. What is the overall attack rate in persons who did not eat ice cream?
a. 30%
b. 33%
c. 35%
d. 44%
e. 58%
Rationale:
The attack rate is the number of persons with diarrhea (14 + 9) divided by the
total number of persons who did not eat ice cream (40 + 30).
6. Which of the food items (or combination of items) is most likely to be the
infective item(s)?
a. Pizza only
b. Ice cream only
c. Neither pizza or ice cream
d. Both pizza & icecream
e. Cannot be assumed from the data shown
Among persons eating ice cream, over 70% developed diarrhea regardless of
their pizza consumption (39/52 & 11/15). Among both groups of persons who did
not eat ice cream, each attack rate was equal to or less than 35% (14/40 &
9/30).
, 7. Which of the following reasons can explain why a person who did not consume
the infective food item got sick?
a. They were directly exposed to persons who did eat the infective food item
b. Diarrhea is a general symptom consistent with a number of illnesses
c. There may have been an inaccurate recall of which foods were eaten
d. All of the above
e. None of the above
Rationale: Without knowledge as to the specific agent in this instance, it is also
likely that it can be spread by direct contact with infected persons. Since
diarrhea is a general disease symptom, it is possible that several infectious
agents may be present at this meal or others eaten during the same time
period. Further, information regarding food consumption may have been
collected long after the disease episode. This may have led persons to
incorrectly remember the foods that they consumed.
An outbreak of gastroenteritis occurred at a boarding school with a student enrollment
of 846. Fifty-seven students reported symptoms including vomiting, diarrhea, nausea, &
low-grade fever between 10 p.m. on September 24 & 8 p.m. on September 25. The ill
students lived in dormitories that housed 723 of the students. The table below provides
information on the number of students per type of residence & the number reporting
illnesses consistent with the described symptoms & onset time. Calculate the attack
rate among all students at the boarding school.
1. Calculate the attack rate among all students at the boarding school.
57/846 = The answer is found by dividing the total number of cases (57) by the
total number of students (846). This equals 6.7%.
10. Calculate the attack rates for boys & girls separately.
a. For boys, the attack rate includes all cases (40 + 3) divided by the total number of students who are
boys (380 + 46). The attack rate is 10.1%.
b. For girls, the attack rate includes all cases (12 + 2) divided by the total number of students who are
girls (343 + 77). The attack rate is 3.3%.
11. What is the proportion of total cases occurring in boys?
