This is a retrospective, blinded, external validation study of a novel in-vitro diagnostic (IVD) assay that will include samples that were previously collected from febrile pediatric patients. The investigated assay measures the levels of a few host-related, blood-based, bio-markers that will be integrated into a single score. Based on this score, each patient will be classified into one of three categories: (i) bacterial immune response, (ii) viral immune response, and (iii) marginal immune response. The assay prediction and the patient diagnosis will than be unveiled and compared to determine their level of concordance.

Inclusion Criteria:

The infectious disease samples should also fulfill the following criteria:

• The patient had a peak temperature ≥38°C (AND)
• Symptoms initiated ≤ 7 days before sampling

The non-infectious disease control group will include:

• Patients with a non-infectious disease

Exclusion Criteria:

• Another infection episode during the last 3 weeks before sampling
• Congenital immune deficiency (CID)
• A proven or suspected HIV-1, HBV, HCV infection
• Active hematological malignancy
• Current treatment with immune-suppressive or immune-modulating
• Other illnesses that affect life expectancy and/or quality of life

The study population will include patients aged 1 month to 18 years of age that were sampled both during emergency department (ED) visit or during hospitalization stay and were diagnosed with an acute infectious disease or a non-infectious disease. All samples are expected to fall into one of the following categories:

1. Patients with an acute bacterial infection
2. Patients with an acute viral infection
3. Patients with an acute mixed co-infection (bacterial and viral)
4. Patients with a non-infectious disease (control group)