Figure 6.18 Geographical development of the swine flu pandemic in the United States, 2009–10 influenza season. Each histogram shows the number of positive influenza virus isolates by Department of Health and Human Services regions reported by WHO/NREVSS Collaborating Laboratories. The preponderance of swine flu A(H1N1), shown in black, as opposed to normal seasonal strains (grey) is evident in all regions.

Source: Data from US Centers for Diseases Control, Influenza Division, available from and