Flow diagram outlining included and excluded publications (year 2001–2010).

<p>A protocol was devised for the systematic study, and the searches were performed in the MEDLINE®, Embase®, Cochrane Reviews and Cochrane Clinical Trials databases. The following inclusion criteria were applied: Infectious agent implicated as a cause or a co-factor of a chronic condition (i.e. disease, disabilities and sequelae that last at least three months); publication in English between 2001 and 2010; study types, i.e. SRs; RCTs, cohort studies, cross-sectional studies, case-control studies, case studies/case series/case reports, literature reviews, or pathological assessments of diseased material. Exclusion criteria were: Infectious agents described as triggers to the chronic condition; non-English language; publication year before 2001; study type, i.e. other studies than those outlines in the inclusion criteria above (including animal studies). In total, 3136 publications were identified in the systematic searches in the databases, and 148 publications were finally included in the analysis. Citations were first screened by two independent reviewers (‘first pass’). Full-text copies of all references that could potentially meet the eligibility were then screened by two independent reviewers (‘second pass’). The data presented in the publications included after this stage were extracted by two independent reviewers.