Abstract: Various embodiments are directed to applications (e.g., classification of biological samples) of the analysis of the count and size of cell-free nucleic acids, e.g., plasma DNA and serum DNA, including nucleic acids from pathogens, such as viruses. Embodiments of one application can predict if a subject previously treated for a pathology will relapse at a future time point. Targeted sequencing (e.g., specifically designed capture probes, amplification primers) can be used to identify DNA across the entire viral genome.