A Study of Ziresovir (AK0529) in High-risk Adult With Respiratory Syncytial Virus (RSV) Infection.

Inclusion Criteria: * Aged 18-85 years (inclusive), regardless of gender. * Diagnosed with RSV infection by antigen or nucleic acid test within 36 hours before first dose. * Have at least one of the following high-risk conditions for RSV infection: Chronic respiratory disease; heart disease; immunocompromised status; other chronic disease; age ≥65 years. * Presence of at least one new LRTD symptom or worsening of a pre-existing LRTD symptom (specifically: cough, expectoration, wheezing, shortness of breath), with at least one individual symptom rated as moderate or severe (score 2 or 3 per RiiQ Symptom Scale). * Time from symptom onset (or worsening of pre-existing symptoms) to first dose ≤5 days. (Onset defined as time of first RSV-related symptom/sign confirmed by investigator; pre-existing symptoms are those due to underlying conditions) Exclusion Criteria: * Diagnosed with influenza or COVID-19 infection within 7 days prior to providing informed consent. * Suspected bacterial co-infection. * Received any potentially anti-RSV agents (e.g., ribavirin, long-acting interferon) within 5 half-lives prior to the first study dose, or plans to use such agents during the study. * Received prohibited medications, or plans to use prohibited medications during the study. * Has a severe or unstable gastrointestinal, hepatic, renal, hematologic disorder, or malignancy. * Has a known allergy, hypersensitivity, or intolerance to AK0529 or to any excipients in the AK0529 or placebo formulation. * Female participants with a positive pregnancy test result, or who are breastfeeding. * Refusal to use effective contraception during the trial period.