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Frequency of porcine influenza virus infection in selected swine herds (top) and individual animals (bottom) sampled in northwest Germany, March 2011 to March 2012. Filled bars, absolute numbers of herds/animals sampled (left y axis); hatched bars, absolute numbers of herds/animals that tested positive (right y axis); shaded bars, percentage of herds/animals that tested positive (right y axis).
