Viruses are intracellular parasites requiring living cells to replicate. Cell culture systems are live cell monolayers that support the growth of viruses. Most laboratories purchase commercially available cell cultures in vials or tubes. The cell culture monolayers are supported by a liquid nutrient medium which serves three general purposes: short term survival as during specimen transport, long term survival such as when cell lines are passaged over time and rapid cell proliferation. The specific liquid medium and cell line is chosen based on the virus requirements. When grown in cell culture, most viruses are detected by their production of cytopathic effect (CPE) which is characteristic and visible (by microscopy) morphological alterations in the cell line due to viral infection.