Fever in infants younger than 3 months is a common reason for emergency department visits and is associated with a significant risk of serious bacterial infections. Because it is difficult to distinguish bacterial from viral infections at presentation, management is often aggressive and includes invasive procedures, hospitalization, and empiric antibiotic therapy.

Despite advances in molecular diagnostics, the etiology of fever remains unidentified in a substantial proportion of cases. This study aims to assess the presence of pathogenic viruses in respiratory and intestinal samples from febrile infants younger than 3 months compared with afebrile controls, and to explore associations with clinical, biological, environmental, and socio-economic factors

Fever in infants under three months of age is a high-stakes clinical condition because severe bacterial infections occur in up to 20-25% of cases, while clinical signs alone cannot reliably distinguish bacterial from viral illness. Due to immune immaturity, management is often aggressive, involving hospitalization, lumbar puncture, and intravenous antibiotics. Although molecular diagnostics (multiplex PCR), bacterial biomarkers (CRP, procalcitonin), and clinical algorithms have improved care, approximately one quarter of cases still lack a confirmed etiology-most often because viral infections are difficult to definitively establish.

This research project aims to improve etiological diagnosis in febrile young infants by systematically evaluating multiplex molecular tests and novel host-response biomarkers (including interferon-induced proteins) using minimally invasive nasal swabs. By correlating these results with final clinical diagnoses-classified as confirmed or probable viral or bacterial infections-the study seeks to clarify the role of these diagnostic tools in early infancy. Seasonal variations as well as environmental and socio-economic factors will be analyzed. A biological sample collection will be constituted for future analyses.The ultimate goal is to enhance diagnostic precision, reduce unnecessary hospitalizations and antibiotic exposure, and optimize the management of febrile infants.

Inclusion criteria :

For participants :

• Age < 3 months
• Fever ≥38°C confirmed in pediatric emergency department

For control group :

• Age < 3 months
• Children requiring general anesthesia or managed in the pediatric emergency department, or during hospitalization or consultation, for a non-infectious condition requiring venipuncture

Exclusion criteria :

For participants :

• Lack of parental/legal guardian consent
• Lack of affiliation with a social security scheme
• Antibiotic treatment within 8 days prior to inclusion

For control group :

• Lack of parental/legal guardian consent
• Lack of affiliation with a social security scheme
• Antibiotic treatment within 8 days prior to inclusion
• Infectious episode within 8 days prior to inclusion

Viral infections are epidemic and strongly seasonal. Therefore, we plan to include children during both the autumn-winter and spring-summer periods. Two groups will be constituted:

Period A (spring-summer): May 2 to August 31 - 50 febrile infants included. Period B (autumn-winter): October 1 to February 28 - 50 febrile infants included.