This Centers for Disease Control and Prevention (CDC) scientist was shown performing a virus plaque assay, to determine the amount of virus in the cell culture isolates. She adds liquid, and then removes it, to see which cells are stained. Cells that are stained blue, or purple, have not been killed by virus. The dead cells are represented by the unstained plaques, which appear as clear open circles that she can then count. The number of plaques observed, is used to determine the amount of virus present.