Common source
100
1
Number of new cases
80
60
40
20
2
3
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Days
Propagating epidemic
Number of new cases
30
25
20
15
10
5 0
1 |
3 |
5 |
7 |
9 |
11 |
13 |
15 17 19 |
21 23 25 27 29 31 33 |
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|
|
|
|
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Days |
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Figure 10 . Temporal distribution of common source vs propagating epidemic
A propagating epidemic curve may appear similar to a common source epidemic curve if the infectious disease has a very short incubation period . Likewise , a common source epidemic curve could appear as propagating with repeated introduction of a toxin over a prolonged period of time .
The index case is circled in red in the propagating epidemic curve ( Figure 10 ). Finding this case is important in identifying the source . In some outbreaks the index case may be an individual case and the time period to the main peak will be the incubation period .
4
5
6
7
8
9
10
29