Inclusion Criteria for main study population :

• Current disease duration ≤ 7 days
• Temperature ≥ 37.8°C (100°F) or tactile fever, noted at least once within the last 7 days
• Clinical suspicion of bacterial or viral respiratory tract infection (RTI)

Exclusion Criteria for main study population:

• Systemic antibiotics taken up to 48 hours prior to presentation
• Outpatient steroids taken within 48 hours prior to presentation
• Suspicion and/or confirmed diagnosis of infectious gastroenteritis/colitis
• Inflammatory disease
• Congenital immune deficiency (CID)
• A proven or suspected infection on the presentation with Mycobacterial, parasitic or fungal (e.g., Candida, Histoplasma, Aspergillus) pathogen
• Human immunodeficiency virus(HIV), Hepatitis B Virus (HBV), or Hepatitis C Virus (HCV) infection (self-declared or known from medical records)
• Major trauma and/or burns in the last 7 days
• Major surgery in the last 7 days
• Pregnancy - Self reported or medically confirmed
• Active malignancy - Cancer diagnosed within the previous six months, recurrent, regionally advanced, or metastatic cancer, cancer for which treatment had been administered within six months, or hematological cancer that is not in complete remission.
• Current treatment with immune-suppressive or immune-modulating therapies, at some point in the past 10 days
• Hemodynamically unstable (require life-saving interventions such as vasopressors)
• Patients transferred from another facility who already have a differentiated respiratory illness (known diagnosis e.g., culture positive results)
• Consider unsuitable for the study by the study team

Inclusion Criteria for Subgroup:

• Written informed consent must be obtained from the patient or his/her legal guardian
• Current disease duration ≤ 7 days
• Clinical suspicion of bacterial or viral sepsis based on 2 or more SIRS criteria OR Clinical suspicion of bacterial or viral respiratory tract infection (RTI) AND temperature ≥ 37.8°C (100°F) or tactile fever, noted at least once within the last 7 days

Patients fulfilling one or more of the following exclusion criteria from the main group are eligible for the subgroup cohort:

• Systemic antibiotics taken up to 48 hours prior to presentation
• Outpatient steroids taken within 48 hours prior to presentation
• Suspicion and/or confirmed diagnosis of infectious gastroenteritis/colitis
• Inflammatory disease (e.g., IBD, SLE, RA, other vasculitis)
• Congenital immune deficiency (CID)
• A proven or suspected infection on the presentation with Mycobacterial (e.g., MAC, MABC), parasitic or fungal (e.g., Candida, Histoplasma, Aspergillus) pathogen
• HIV, HBV, or HCV infection (self-declared or known from medical records)
• Major trauma and/or burns in the last 7 days
• Major surgery in the last 7 days
• Pregnancy - Self reported or medically confirmed
• Active malignancy of a solid tumor - Cancer diagnosed within the previous six months, recurrent, regionally advanced, or metastatic cancer, cancer for which treatment had been administered within six months, or hematological cancer that is not in complete remission
• Current treatment with immune-suppressive or immune-modulating therapies, at some point in the past 10 days
• Hemodynamically unstable (require life-saving interventions such as vasopressors)
• Patients transferred from another facility who already have a differentiated respiratory illness (known diagnosis e.g., culture positive results)