Specifically, a confluent monolayer of host cells is infected with the virus at varying dilutions and covered with a semi-solid medium, such as agar or carboxymethyl cellulose, to prevent the virus infection from spreading indiscriminately. A viral plaque is formed when a virus infects a cell within the fixed cell monolayer.[1] The virus infected cell will lyse and spread the infection to adjacent cells where the infection-to-lysis cycle is repeated. The infected cell area will create a plaque (an area of infection surrounded by uninfected cells) which can be seen with an optical microscope or visually (pouring off the overlay medium and adding a crystal violet solution for 15 minutes until it has colored the cytoplasm, gently removing the excess with water will show uncolored the location of dead cells[2]).