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:This finding may account for the apparent persistence of previously unknown parasite strains in populations that are severely affected by HIV-1 (Pieniazek et al. 1999; Gramiccia, 2003; Pratlong et al. 2003).
Category: