Patient description: A 17 year old boy complains of vomiting, non-bloody diarrhea, abdominal pain, fever, chills and loss of appetite for the past 3 days. He ate a salad at a restaurant prior to his diarrhea onset. Physical exam was remarkable for pallor, jaundice, and diffuse abdominal tenderness. Lab results were as follows: Hemoglobin: 9.7 g/dL Platelet: 110,000 /cu.mm Creatinine: 3.6 mg/dL blood urea nitrogen (BUN): 73 mg/dL direct bilirubin: 2.4 mg/dL lactate dehydrogenase (LDH): 881 IU/L (normal: 110-265 IU/L) Peripheral blood smear showed a moderate number of schistocytes and helmet cells. Shiga-like toxin-producing E. coli (STEC) stx1/stx2 were found in stools. He has no other underlying disease and he is not on any medications.

Clinical trial eligibility criteria: eligible ages (years): 0.0-100.0,  Malaria (Cohort Study) Resident of Kandal, Ekapheap, and Sangkumthmey villages (Thmar Da commune), and no plans to leave the Thmar Da commune for the next 5 years. Willingness to participate in the study as evidenced by informed consent of subjects or his/her parent or guardian, and willingness to come to commune health posts if he/she develops fever or other symptoms of malaria. Individuals of all ages will be enrolled  (Cohort Study) Any condition that in the opinion of the investigator would render the subject unable to comply with the protocol (e.g., psychiatric disease). Any health condition that in the opinion of the investigator would confound data analysis or pose unnecessary exposure risks to study personnel (e.g., individuals who are known to be HIV-infected or to have AIDS). (P. vivax Collection Study) P. vivax malaria (mono-infection). Willingness to participate in the study as evidenced by informed consent of subjects or his/her parent or guardian. Age greater than or equal to 2. Hematocrit greater than or equal to 25% (P. vivax Collection Study) Any condition that in the opinion of the investigator would render the subject unable to comply with the protocol (e.g., psychiatric disease). Any health condition that in the opinion of the investigator would confound data analysis or pose unnecessary exposure risks to study personnel (e.g., individuals who are known to be HIV-infected or to have AIDS). Pregnancy. Prior use of antimalarials during the past 2 months. (Cord and Peripheral Blood Collection Study) Healthy male or female adults greater than or equal to 18 years old or healthy pregnant female adults greater than or equal to 18 years old. Willingness to participate in the study as evidenced by informed consent (Cord and Peripheral Blood Collection Study) Any condition that in the opinion of the investigator would render the subject unable to comply with the protocol (e.g., psychiatric disease). Any health condition that in the opinion of the investigator would confound data analysis or pose unnecessary exposure risks to study personnel (e.g., individuals who are known to be HIV-infected or to have AIDS). (Collection of peripheral blood for identifying and isolating memory B cells) Healthy male or non pregnant female adults greater than or equal to 18 years old. Previous enrollment on the P. vivax collection study. Willingness to participate in the study as evidenced by written informed consent (Collection of peripheral blood for identifying and isolating memory B cells) For the follow-up blood draw (250 mL), symptomatic parasitemia with any species of Plasmodium. For the follow-up blood draw (250 mL), hemoglobin level <9 g/dL. For the follow-up blood draw (250 mL), weight <45 kg. Any condition that in the opinion of the investigator would render the subject unable to comply with the protocol (e.g., psychiatric illness). Any health condition that in the opinion of the investigator would confound data analysis or pose unnecessary exposure risks to study personnel (e.g., individuals who are known to be HIV-infected or to have AIDS)