Cultivation of viruses 
      •  Virologists need to be able to produce the objects of their 
         study, so a wide     range of procedures has been developed 
         for cultivating viruses. Virus  cultivation is also referred to as 
         propagation or growth.
      •  Phages are supplied with bacterial cultures, plant viruses may be 
         supplied with specially cultivated plants or with cultures of 
         protoplasts (plant cells from which the cell wall has been 
         removed), while animal viruses may be supplied with whole 
         organisms, such as mice, eggs containing chick embryos(figure 
         2-1)  or insect larvae. For the most part, however, animal 
         viruses are grown in cultured animal cells.
           Animal cell culture
   Animal cell culture techniques are well developed and most of the 
   cells used are from continuous cell lines derived from humans and 
   other animal species. Continuous cell lines consist of cells that 
   have been immortalized, either in the laboratory or in the body 
   (Figure 2.2); they can be sub cultured indefinitely. 
   • The HeLa cell line is a widely used continuous cell line that was 
    initiated in the middle of the 20th century from cells taken from 
    a cervical carcinoma. Sometimes it is difficult to find a cell line in 
     which a virus can replicate. For many years no suitable cell 
    culture system could be found for hepatitis C virus, but 
    eventually a human hepatoma cell line was found to support 
    replication of an isolate of the virus.