To externally validate the diagnostic accuracy and assess the clinical utility of a host-response based diagnostic tool called ImmunoXpert™, for differentiating between bacterial and viral etiologies in pediatric patients >3 months old with suspicion of Respiratory tract infection (RTI) or Fever without Source (FWS)

Inclusion Criteria:

• Documented peak temperature ≥ 38°C (100.4°F) (AND)
• Symptom duration ≤ 7 days (AND)
• Clinical suspicion of RTI (OR)
• Fever without a clear source (OR)
• Acute gastroenteritis (OR)
• Urinary tract infection

Exclusion Criteria:

• Antibiotic treatment of over 48 hours' duration at time of presentation

• Another episode of febrile infection within the past 2 weeks

• A proven or suspected HIV1, HBV, or HCV infection

• Congenital immune deficiency (CID)

• Active malignancy

• Current treatment with immune-suppressive or immune-modulating therapies, including without limitations:

  • Use of high dose steroids >1 mg/kg/day prednisone or equivalent in the past two weeks Monoclonal antibodies, anti-TNF agents
  • Intravenous immunoglobulin (IVIG)
  • Cyclosporine, Cyclophosphamide, Tacrolimus
  • G/GM-CSF, Interferons

• Other severe illnesses that affect life expectancy and quality of life such as:

  • Moderate to severe psychomotor retardation
  • Post-transplant patients
  • Moderate to severe congenital metabolic disorder

The study population will include eligible subjects from both genders that present to the ED due to suspected RTI, FWS, GE, UTI or non-febrile non-infectious disease (Control). Each of these subjects falls 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 an undetermined disease etiology
5. Patients with a non-infectious disease (control group)