FEBRILE ADULTS- INCLUSION CRITERIA  Age greater than or equal to 15 years and less than or equal to 45 years Participant is willing and they and/or an appropriate guardian/relative/representative is able to give informed consent for participation in the study and a follow-up (telephone) discussion at 14 days  Febrile illness without localising features (see 'exclusion criteria' and 'screening' sections) where fever is defined as:  documented tympanic/rectal temperature of ≥ 38°C or an axillary/oral temperature of ≥ 37.8°C and Reported duration of fever 3-14 days They may have had recent exposure to antimicrobials.  FEBRILE ADULTS- EXCLUSION CRITERIA  The participant may not enter the study if ANY of the following apply: Unable to provide informed consent and no next of kin is willing and able to provide informed consent.  Any history or clinical suspicion of:  Rheumatological or connective tissue disorder (e.g. Rheumatoid arthritis) Autoimmune condition (e.g. Autoimmune Hepatitis) Malignancy Active treatment with immunomodulating medications, or for tuberculosis (pulmonary or extrapulmonary) or any other chronic infection. Pregnancy (breast feeding mothers will NOT be excluded) Hospitalisation within 4 weeks prior to current admission Vaccination within 8 weeks prior to current admission They live outside of the normal/local catchment area for each hospital site Localising signs or symptoms of infection sufficient to diagnose the likely cause of acute febrile illness and thus prevent it from being 'undifferentiated' (see Section 8.4 'Screening and Eligibility').  CONTROLS- INCLUSION CRITERIA  Participant is willing and they are (and in 15-18 year olds, a guardian is) able to give informed consent for participation in the study Age greater than or equal to 15 years and less than or equal to 45 years They live outside of the normal/local catchment area for each hospital site Afebrile (as defined by no reported fever and temperature ≤ 38°C or an axillary/oral temperature of ≤ 37.8°C).  CONTROLS- EXCLUSION CRITERIA  The participant may not enter the study if ANY of the following apply: Unable to provide informed consent and no next of kin (or parent/guardian in the case of a minor) is willing and able to provide informed consent.  Current treatment for or prior history, or clinical suspicion of:  Rheumatological or connective tissue disorder Autoimmune condition Malignancy Active treatment for tuberculosis (pulmonary or extrapulmonary) or clinical suspicion of active tuberculosis. Previous completed treatment is not a reason for exclusion. Active treatment with immunomodulating medications or any other chronic infection. Pregnant (breast feeding mothers will NOT be excluded) Hospitalisation within 4 weeks of recruitment Vaccination within 8 weeks prior to recruitment Antimicrobial use within 4 weeks of recruitment They live outside of the normal/local catchment area for each hospital site Participant reports feeling more unwell than usual on the day of enrolment.  EXPLORATORY AIMS- PAEDIATRIC CRITERIA; INCLUSION CRITERIA  Age greater than or equal to 2 years and less than 15 years As above for adult participants.  EXPLORATORY AIMS- PAEDIATRIC CRITERIA; EXCLUSION CRITERIA  Parent/guardian is unwilling, and/or patient aged 8 to 14 years is unwilling to assent to provide informed consent. As above for adult participants.