A plaque-forming unit (PFU
) is a measure of the number of particles capable of forming plaques
per unit volume, such as virus
particles. It is a functional measurement rather than a measurement of the absolute quantity of particles: viral particles that are defective or which fail to infect their target cell will not produce a plaque and thus will not be counted. For example, a solution of Tick-borne encephalitis virus
with a concentration of 1,000 PFU/µl
indicates that there are 1,000 infectious virus particles in one microliter of solution.