Accurately assessing the exposure of a population to a particular virus is difficult because the tools for doing so do not account for the fact that many viruses comprise multiple circulating strains, or the fact that people can be vaccinated or naturally immune, among other factors. Using influenza as a model, researchers have developed a new technique that overcomes many of these roadblocks, and they say the tool may be useful for better assessments of exposure to a variety of viruses, including the ones that cause COVID-19 and pneumonia.