This is a prospective cohort study where HIV-infected and uninfected children will be enrolled between 6 weeks and 9 months of age and followed to the age of 21 months. All HIV-infected children will be given trimethoprim-sulfamethoxazole (TMP-SMX) prophylaxis as of 6 weeks of age. HIV-uninfected children born to HIV-infected mothers will be given TMP/SMX prophylaxis for the duration of breastfeeding and then randomized to the continuation of TMP/SMX or discontinuation of TMP/SMX prophylaxis. HIV-uninfected children born to HIV-uninfected mothers will not be given TMP/SMX prophylaxis. Study participants will be followed for all of their health care needs in a designated study clinic. All mother-child pairs will receive a basic care package including insecticide-treated bednets (ITNs) at enrollment. All HIV-infected mothers and children will receive antiretroviral therapy if eligible according to standardized World Health Organization (WHO) criteria. Study participants 4 months of age or older and at least 5 kg will be randomized to treatment with artemether-lumefantrine (AL) or dihydroartemisinin-piperaquine (DP) at the time of their first diagnosis of uncomplicated malaria. Study participants will receive the same antimalarial treatment regimen for all future episodes of uncomplicated malaria. Study participants less than 4 months of age or less than 5 kg diagnosed with malaria and all episodes of complicated malaria will be treated with quinine in accordance with local guidelines.

The investigators will test the hypotheses that:

1. TMP/SMX prophylaxis is highly effective in preventing malaria in both HIV-infected and HIV-uninfected children
2. The use of TMP/SMX prophylaxis is associated with an increased risk of infection with malaria parasites containing antifolate resistance-conferring mutations.
3. The use of antiretroviral (ARV) drugs is associated with a decreased incidence of malaria.

4. The efficacy, safety, and tolerability of AL and DP for the treatment of uncomplicated malaria differ.

   In 2008, we received approval and funding to extend the trial until 2012. We are now following all children through 5 years of age. First randomization to continue or discontinue TMP/SMX prophylaxis in our HIV-exposed population occurs 6-8 weeks after cessation of breastfeeding when HIV status can be confirmed as negative by DNA PCR. A second randomization occurs at 2 years of age in our HIV-exposed participants. At that point all HIV-exposed children who were originally randomized to continue TMP/SMX prophylaxis are again randomized to either immediately discontinue TMP/SMX prophylaxis or continue prophylaxis until age 4 years. All children will be off TMP/SMX between 4 and 5 years of age.

   We have also added an additional hypothesis to test during the study extension:

5. Prolonged TMP/SMX prophylaxis will result in an increased incidence of malaria in children in the year immediately following cessation of prophylaxis compared to children who have not used prophylaxis for over a year and those who have never been on prophylaxis.