Part 2 will consist of up to 3 cohorts: 1) 160 mg BCX4208 or placebo; 2) 240 mg BCX4208 or placebo; and 3) 320 mg BCX4208 or placebo. Unlike Part 1, Part 2 is a dose-escalation design whereby each of the cohorts will be enrolled sequentially, following review of the efficacy, safety, and tolerability data of the previous cohort. Enrollment into each cohort during Part 2 will be in a 3:1 ratio of BCX4208 to placebo such that 15 subjects will be randomized into each of the BCX4208 groups (total of 45 subjects) and 15 subjects will be randomized to placebo.