Next-generation sequencing (NGS) has been applied to metagenomic studies on viral diversity in clinical samples for many years,but the overall detection rate and genome coverage of low abundant viruses is very poor,even combined with additional viral particle preparation or pre-amplification protocols.The viral copy number in clinical samples can vary by many orders of magnitude,for instance,from less than 100 to more than a million per ml plasma over the course of an infection and treatment process.Traditional diagnostic assays are designed to determine levels of viral antibodies or copy numbers for viral DNA/RNA using ELISA or qPCR-based methods,respectively;neither have sufficient sensitivity to consistently detect viral signal at the lower extremes.