Plaque forming unit

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.