Virus infectivity

In order to produce new virions, viruses need to be capable of replicating in a susceptible host cell. The  replication cycle  is accompanied by a number of biochemical and morphological changes within the  cell (e.g. cell rounding or fusion), referred to as the cytopathic effect  (CPE), which usually results in the  death of the  cell within a defined time  post infection (p.i.). A virus will give rise to a particular CPE in a given cell type and this can be used when attempting to identify an unknown virus or to quantify a known virus. The appearance and detection of CPE regularly forms the basis of infectivity assays designed to determine the number of infectious units of virus per unit volume, and this measure of infectivity is referred to as the titer. An infectious unit is thought of as being the smallest amount of virus that will produce a detectable biological effect in the assay and the name given to that unit depends on the assay used, as different assays are not necessarily equivalent. Some  infectivity assays are  quantal, in that they  use  an  ‘all or nothing’ approach, such as the  tissue culture infective dose  50 ( TCID50) assay,  while the plaque assay  is focal  in that it detects and  enumerates discrete foci of infection. Both assays provide accurate quantitative information regarding the virus under test, but the focal plaque assay provides definitive quantification of the actual numbers of infective virions within a sample.