Patients (children and/or adults) presenting with (a history of) fever are asked permission (informed consent and/or assent) to draw additional blood samples (along with routine care samples, so no need for additional venipuncture), and for the long term storage of these samples (whole blood and plasma) in a freezer together with left-over serum and urine samples. They are also asked permission for the collection of coded data including basic demographic, clinical and laboratory data.