In this study, two groups of volunteers will be exposed to malaria through the bites of infected mosquitoes. In one group, volunteers will be randomly assigned to one of 5 arms. Each of these arms will receive a different dose of Malarone®, a drug known to prevent malaria when taken daily. Each of these doses will be lower than the maximum approved dose of this medicine. The other group will not be treated with any drug that could prevent symptoms or infection.