Volume 19, Number 6—June 2013
Research
Active Surveillance for Influenza A Virus among Swine, Midwestern United States, 2009–2011
Figure 2

Figure 2. . Frequency distribution of number of nasal swab samples positive for influenza virus by real-time reverse transcription PCR, per group (total 540 groups of pigs), midwestern United States, June 2009–December 2011.