Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:PCV-1 was first detected as a contaminant of the porcine kidney PK-15 cell line [4] whereas PCV-2 has been associated with postweaning multisystemic wasting syndrome (PMWS) [5, 6], porcine dermatitis and nephropathy syndrome (PDNS) [7], proliferative necrotizing pneumonia [8], and reproductive disorders [9], all of them included by the term porcine circovirus associated diseases (PCVAD) [10].
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