syringe with red liquid

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