Inclusion Criteria:  In order to be eligible to participate in this study (i.e., to be enrolled in the relevant institutional clinical care pathway), an individual must meet all of the following criteria:  Aged 9.0 months to <21 years at the time of informed consent.  Evidence of high-risk STEC infecting pathogen defined by any of the following:  Bloody diarrhea within the preceding 7 days  Positive STEC culture OR Positive antigen/polymerase chain reaction test for toxin/gene type not otherwise specified OR  Bloody or Non-bloody diarrhea within the preceding 7 days  •Presumptive diagnosis of HUS  (meeting all 3 HUS criteria - anemia, thrombocytopenia, and renal insufficiency) OR  Non-bloody or no diarrhea  Positive STEC culture for high-risk strain (i.e., O103, O104, O111, O113, O121, O145 or O157) OR Positive antigen/polymerase chain reaction test Stx2 toxin/gene  Exclusion Criteria:  All individuals meeting any of the exclusion criteria at baseline will be excluded from study participation.  Presence of Advanced HUS defined by:  Hematocrit <30% AND Platelet count <150 x 103/mm3 AND  Creatinine > 2.0 mg/dL (177 µmol/L)  The presence of only 1 or 2 of these criteria will not result in patient exclusion, regardless of how close the 3rd criterion is to meeting the exclusion criteria. Prior episode of HUS or diagnosis of atypical HUS. Chronic disease limiting fluid volumes administered (e.g. impaired liver or cardiac function, chronic lung disease). Evidence of anuria (i.e., no urine output for > 24 hours). Hypoxemia requiring oxygen therapy Hypertensive emergency Greater than or equal to 10 days since onset of diarrhea or if no diarrhea then the onset of other symptoms. Patients with known CAKUT (congenital anomalies of the kidney and urinary tract), including kidney anomalies, renal hypo dysplasia, ectopic kidney, horseshoe kidney, multicystic dysplastic kidney, hydronephrosis, duplicated collecting system, ureteropelvic junction obstruction, megaureter, vesicoureteral reflux, and posterior urethral valve. Patients with known pregnancy Patients or caregivers with language barriers impairing appropriate conduct of the study protocol.