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Chapter 2-4
Which of the following is a condition which may occur during the incubation period?
• Transmission of infection
The incubation period is defined asthe interval from receipt of infection to the time of onset of
clinical ill...
distribution of this document is illegal student consult chapter 2 4 which of the following is a condition which may occur during the incubation period • transmission of infection the incubation peri
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Student Consult
Chapter 2-4
Which of the following is a condition which may occur during the incubation period?
• Transmission of infection
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.
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, and fluid-filled blisters. One month after the boy returned
to school, none of his classmates had been infected by VZV. The main reason was:
• Contact was after infectious period
• Subclinical infections were not yet detected
• Disease was endemic in the class
The disease is spread by contact with an infected individual who can transmit the agent (VZV) to
immunologically naive persons during the incubation period and 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.
The ability of a single person to remain free of clinical illness following exposure to an
infectious agent is known as:
• Hygiene
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, • Vaccination
• Herd immunity
• Immunity
• Latency
Immunity is the capacity of a single individual to avoid disease susceptibility when exposed to
an infectious agent. Herd immunity is a population characteristic. For certain diseases, individual
immunity can be acquired by vaccination, but this is not true for all infectious diseases.
Which of the following is characteristic of a single-exposure, common-vehicle outbreak?
• Long latency period before many illnesses develop
• There is an exponential increase in secondary cases following initial exposures
• Cases include only those who have been exposed to sick persons
• The epidemic curve has a normal distribution when plotted against the logarithm of time
• Wide range in incubation times for sick individuals
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.
What is the diarrhea attack rate in persons who ate both ice cream and pizza?
• 39/52
• 21/70
• 39/67
• 51/67
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, • none of the above
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 and pizza. Of these 52 persons, 39 developed diarrhea.
What is the overall attack rate in persons who did not eat ice cream?
• 30%
• 33%
• 35%
• 44%
• 58%
(14+9)/(40+30)= 33%
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).
Which of the food items (or combination of items) is most likely to be the infective item(s)?
• Pizza only
• Ice cream only
• Neither pizza nor ice cream
• Both pizza and ice cream
• Cannot be assumed from the data show
Among persons eating ice cream, over 70% developed diarrhea regardless of their pizza
consumption (39/52 and 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 and 9/30).
Which of the following reasons can explain why a person who did not consume the infective
food item got sick?
• They were directly exposed to persons who did eat the infective food item
• Diarrhea is a general symptom consistent with a number of illnesses
• There may have been an inaccurate recall of which foods were eaten
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, • All of the above
• None of the above
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, and
low-grade fever between 10 p.m. on September 24 and 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 and the number reporting
illnesses consistent with the described symptoms and onset time. Calculate the attack rate
among all students at the boarding school.
57 total cases/ 846 total students=6.7%
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