A technique called VirScan can, in a single step, survey most of the viruses to which a person has been exposed over a lifetime by infection or vaccination. Stephen J. Elledge of Harvard Medical School and coworkers developed the technique and used it to determine exposures of nearly 600 people in several countries (Science 2015, DOI: 10.1126/science.aaa0698). Their analysis shows that each person had been exposed on average to about 10 viral species, with some species representing multiple strains. VirScan is carried out by using a collection of bacteriophage-expressed peptides from all human viruses to screen for corresponding antibodies in blood samples. The researchers estimate that a VirScan test would cost about $25. VirScan can be used to help study the effects on human health and disease of interactions between viruses and the immune system “and could easily be expanded to include other human pathogens such as bacteria, fungi, and protozoa,” the researchers write.