A free phage, a phage infection (singly or multiply of the same bacterium), or even a phage-infected bacterial arrangement or aggregation, all as give rise to a single plaque upon plating.
Equivalent to a plaque-forming unit and also described as an infectious center.
This is the definition from Adams (1959), p. 440, "A plaque-forming particle that may be either a productively infected bacterium or a free phage particle. When one or the other is meant, the term infective center should not be used." That is, infective center is a broader description versus the narrower, specific cases of infected bacterium or free phage.
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