Introduction to Epidemiology 6th Edition by Ray M Merrill

Exercise 16

Develop and construct a bar chart of the epidemiological curve for the shigellosis outbreak using the data in Table 1. Identify the index case and date, and explain who the index case was. What date was the water chlorinated? What did chlorinating the water do to the 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 4

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.