1. 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.

2. 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.

3. 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.

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?veloped 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).