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:West Nile virus (WNV) is a mosquito-borne pathogen that began circulating in the Caribbean Basin in 2001.(1) Ecological studies of WNV in Europe, Asia, the Middle East, and the United States have shown that Culex (Culex) mosquitoes
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