The Treatment Period begins on Day 1. Subjects are to arrive at the study clinic on Day 1 after an overnight fast. After a final review of eligibility criteria, pre-dose vital signs assessments, and pre-dose BCX4208 pharmacokinetic (PK) blood draw have been performed, subjects will be randomized and administered the first dose of study drug. Subjects will remain in the study clinic for Hour 2, Hour 4, and Hour 8 assessments and will return to the study clinic for efficacy and safety evaluations on Days 2, 8, 15, and 22.