Baphicacanthus cusia, also known as "Qing Dai" in traditional Chinese medicine (TCM), has been historically used for its antiviral properties. Traditional texts and materia medica describe its application in the treatment of viral infections such as hepatitis, measles, mumps, and influenza. The use of B. cusia is based on its reputed ability to "clear heat" and "detoxify," concepts in TCM that correspond to combating fever and infectious diseases. Extracts from the plant, especially indigo naturalis, are often used externally and internally in various TCM formulations. Modern scientific research has begun to investigate these traditional claims. Some in vitro and animal studies have reported that extracts of Baphicacanthus cusia possess antiviral activity against certain viruses, such as influenza A and hepatitis B virus. The active components, including indirubin and tryptanthrin, have demonstrated inhibitory effects on viral replication in laboratory settings. However, robust clinical trials in humans are lacking, and the quality of existing evidence is limited. In summary, the use of Baphicacanthus cusia for supporting or treating viral infections is primarily justified by traditional use, with some preliminary scientific data suggesting potential antiviral activity. Nevertheless, the overall strength of evidence is modest, and more rigorous clinical studies are needed to confirm its efficacy and safety for viral infections.