Seasonal and Age-Specific Patterns of Viral and Bacterial Infections in Febrile Infants ≤ 90 Days Presenting to a Paediatric Emergency Department.

AimFebrile infants ≤ 90 days are at increased risk of serious and invasive bacterial infections (SBIs/IBIs), although viral illnesses account for most febrile episodes. Current management algorithms prioritize bacterial risk stratification, while viral epidemiology is inconsistently integrated into pathways. Aim of this study is to describe age-specific bacterial and viral etiologies, seasonality, temporal trends, and co-infections in febrile infants presenting to a tertiary paediatric emergency department.MethodsRetrospective observational study including infants ≤ 90 days evaluated between 2018 and 2025. Infants were stratified into three age groups (≤ 28, 29-60, 61-90 days). Bacterial and viral etiologies, co-infections, seasonal distribution, and temporal trends were analysed.ResultsAmong 1529 infants, SBI represented 15% of cases. IBI was age-dependent, peaking in neonates ≤ 28 days, whereas viral infections predominated across all groups. Approximately 10% of SBIs occurred with documented viral co-infection. Bacterial infections showed no seasonal pattern, while viral infections demonstrated seasonality, with winter respiratory peaks and summer enterovirus/parechovirus circulation. Viral detections declined during 2020-2021 and rebounded in 2022 whereas bacterial rates remained stable.ConclusionsViral etiologies display pronounced seasonality, whereas bacterial infections remain non-seasonal. Viral positivity does not exclude SBI and urinalysis should remain routine. An epidemiology-aware approach may support without replacing bacterial risk stratification.