Viral plaque assays are a simple yet ingenious way of enumerating the number of viruses within a sample. This technique requires introducing the viruses into a nutrient medium (typically agar) in which a culture of bacteria or other cells has been grown.
In this approach, the viruses spread through the cell culture, invading and destroying the cells in question. The viruses thus generate zones of cell destruction, which we call plaques. These plaques can be detected visually, but they may not necessarily be visible to the naked eye. Sometimes, they can only be seen through the assistance of other techniques such as staining, microscopy, hemadsorption or immunofluorescence.
Naturally, this technique is not foolproof, since plaques can overlap and even occlude one another. Statistical methods must then be used to estimate the actual number of viruses introduced into the sample.