The vertical dotted lines indicate the median of median SSU rRNA abundances in human/animal (red dotted line) and environmental (blue dotted line) unenriched metagenomes, as a reference. The two medians are used to calculate the enrichment rate of each virome with respect to the reference metagenomes. The frequency of enrichment levels of all the 1,977 viromes that passed quality control is represented in the inset histogram. The histogram on the right side shows the number of reads (bar height) and the number of samples (to the left of the bar) in each dataset. Datasets are grouped by type (environmental or human/animal). Datasets within the same group are ordered by median abundance. References to each dataset are provided in Supplementary Tables 1 and 2. Error bars in the right barplot show the 95% confidence intervals. Boxes show the quartiles of each dataset, the central line in each box indicates the median, while whiskers extend to show data points within 1.5 interquartile range. Data points, including outliers, are overlaid on the boxes.