person holding syringe near container

In this 2019 photo, a Centers for Disease Control and Prevention (CDC) scientist removes cell growth medium from a 6-well plate in preparation for a plaque assay, which is a test that allows scientists to count how many flu virus particles (virions) are in a mixture. The next step is to add flu virus to each of the wells to allow the virus to enter the host cells that are attached to the bottom of the plate. Photographer James Gathany