This study will be conducted in Siaya County in Nyanza Province, western Kenya. Healthy children aged 5 months through 9 years of age living within approximately 10 km of the study clinic(s) (Siaya County Referral Hospital, or Wagai dispensary, a government health facility in Wagai division) will be eligible for participation in Part 1; healthy infants aged 5 months - 12 months inclusive will be eligible for Part 2.