General risk factors:
(1) age (young children <= 5 years of age, or the elderly >= 70 years of age)
Gastric and intestinal related factors:
(1) achlorhydria
(2) regular antacid use
(3) gastrectomy or gastroenterostomy
(4) impaired small and large bowel motility
(5) schistosomiasis
Hematologic risk factors:
(1) hemolytic anemia, including sickle cell anemia
(2) acute hemolysis associated with malaria and bartonellosis
Immunologic risk factors:
(1) immunosuppressive therapy
(2) malnutrition
(3) congenital immune deficiency syndromes
(4) acquired immune deficiency, including HIV disease
(5) malignancies, including leukemia and malignant lymphoma
(6) collagen vascular diseases
