Certain aspects of the present disclosure generally relate to systems and methods for determining viruses. For instance, some aspects are directed to systems and methods for determining viruses using a partitioning system. Within the partitioning system, the virus may partition into one or more phases. In some cases, a virus-binding moiety facilitates partitioning of the virus. The phases may be assayed to determine the virus based on, e.g., quantitative or qualitative assessments of the distribution of virus-binding and/or signaling moieties. The virus-binding moiety may be attached to particles that may form a complex around a virus. The complex may be detectable without a signaling moiety (e.g., as a color change) in some embodiments. In some cases, more than one virus may be determined. For example, a virus-binding moiety may substantially alter the partitioning behavior of one virus or complex, relative to another, by being selective for the first virus.