Introduction to Epidemiology 6th Edition by Ray M Merrill

Exercise 17

Develop a bar graph of the secondary attack rate for the shigellosis epidemic using the data from Table 1. Explain why the secondary epidemic curve is different than that of the primary epidemic curve.

Table 1 Date of Onset of Secondary Attack Rates of Gastrointestinal Illness in Faculty and Staff, Van Buren Middle School, Stockport, Iowa, 1972

Date of Illness Onset	Number of Cases	Date of Illness Onset	Number of Cases
November		November
7	1	22	8
8	0	23	2
9	0	24	3
10	1	25	13
11	3	26	4
12	1	27	4
13	4	28	2
14	0	29	9
15	0	30	8
16	5	December
17	2	1	3
18	5	2	0
19	3	3	5
20	7	4	0
21	3	5	0

Step 1 of 3

An epidemic curve is a representation of the number of cases (diseased individuals) observed during an outbreak or epidemic against time. This curve displays the course of the epidemic starting from the date or time of the epidemic onset. The form of the bar graph helps in the formulation of a hypothesis that describes the nature of the disease and the mode of transmission.