<prompt> Plaque formation in virus is done for:Isolation and typing of viursesCloning separation of specific virusesDetermining infectivity of virusAssessing multiplication of virus  <response>: Ans. c Determining infectivity of virus Ref. Ananthanarayan 8e, p 435, 9e, p 437 By By Quantal assay Quantitative Infectivity assay of virus Electron Hemagglutination Indicates presence or Measure actual number of infectious microscopy inhibition absence of infectious viruses. paicle in inoculum Titre is usually expressed as 50 percent infectious dose ID50 per ml calculated by statistical methods e.g. Reed and Muench Plaque assay or monolayer cell culture Transformation assay For oncogenic virus Pockassay on choronic allan-toic membrane CAM e.g. variola, vaccina herpes virus monkey pox, cow pox Plaque assay: Each infectious paicle give rise to a localized focus of infected cells that can be seen with naked eye. Such foci are knows as plaque and each plaque indicates an infectious virus. Titre is usually expressed as 