Scientist examining the results of a plaque assay, which tests how many flu virus particles (virions) are in a mixture. To carry out a plaque assay, host cells are grown in the wells (circles) on a plate, before having a diluted solution containing viruses added to them. Uninfected cells (purple) are stained so that each clear spot (plaque) represents a cell infected with a virus. Each plaque represents one virion, which can be used to estimate the quantity of infectious virions in the original virus solution.