Figure 1.

Schematic diagram of the course of illness and clinical iceberg of upper respiratory infections in a population, and examples of surveillance systems targeting each stage. The exact proportion of infections falling into each category, and the specificity of surveillance data to influenza will vary at different times and in different settings.

Cheng et al. BMC Public Health 2009 9:339   doi:10.1186/1471-2458-9-339
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