Part 2 of the study will evaluate the safety, tolerability, and pharmacokinetics of 7 days of daily dosing with BCX4430 versus placebo in healthy subjects. Up to 4 ascending dose cohorts will be treated in a sequential manner. Ten subjects will be treated with study drug per dose cohort (8 subjects will receive BCX4430 and 2 subjects will receive placebo per cohort). The planned doses and dosing regimens for each of the Part 2 cohorts will be determined based upon data collected during Part 1.