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:…co-infection to be associated with increased variability of parasite strains (Chicharro et al. 2003; Mathis et al. 2005) or the discovery of previously unknown strains (Pieniazek et al. 1999; Gramiccia, 2003), but the origin and epidemiological characteristics of these strains are often unknown.
Category: