In virology, 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 1 µl of the solution contains enough virus particles to produce 1000 infectious plaques in a cell monolayer, but no inference can be made about the relationship of pfu to number of virus particles.
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