This randomized, double-blind, multiple-dose, placebo-controlled study will evaluate the safety, tolerability and efficacy of balapiravir in adult male patients with confirmed dengue virus infection whose symptoms began within the 48 hours preceding the first administration of balapiravir. Patients will be randomized to receive either balapiravir or placebo, orally twice daily for 5 days. Anticipated time on treatment as in-patient is 7 days, with an out-patient follow-up to week 12. Target sample size is <200