woman holding laboratory appratus

Scientist examines the result of a plaque assay, which is a test that allows scientists to count how many flu virus particles (virions) are in a mixture. To perform the test, scientists must first grow host cells that attach to the bottom of the plate, and then add viruses to each well so that the attached cells may be infected. After staining the uninfected cells purple, the scientist can count the clear spots on the plate, each representing a single virus particle.