Figure 4 is a histogram showing the number of stools that tested positive for Shigella each month between 2010 and 2014, by type of test performed. The types of test include polymerase chain reaction, culture, or both polymerase chain reaction and culture. A line shows the total percent positivity for Shigella. Prior to late 2013, all specimens were positive by culture alone. After polymerase chain reaction was introduced in late 2013, the total number of tests positive and percent positivity increased dramatically, and approximately 85% of specimens were positive by polymerase chain reaction alone.