Certain aspects of the present disclosure generally relate to systems and methods for determining viruses such as coronaviruses. For instance, some aspects are directed to systems and methods for determining viruses using a partitioning system. Within the partitioning system, free RNA or other nucleic acids may preferentially partition into one phase, while intact viruses may be present in the other phase or in both phases. Accordingly, in some cases, free RNA or other nucleic acids may be preferentially removed, e.g., as compared to intact RNA or other nucleic acids present within a virus. In some cases, the phase containing intact viruses can be determined to determine the infectiousness, e.g., of a sample arising from a subject. This may be useful, for example, for distinguishing subjects who are capable of spreading an infection from those who are not infectious.