Viruses can also be reared in cell (tissue) cultures.  These cultures are produced by separating animal cells from a particular tissue and allowing them to grow an an appropriate medium.  Viruses can then be introduced to the culture and allowed to replicate.

Cell cultures are either diploid (derived from plant or animal embryonic cells and having the ability to divide only about 100 times before cell death) or continuous (derived from hyperplastic tumor cells that continue to reproduce, such as the HeLa cell line, derived from Mrs. Henrietta Lacks, who died of cervical cancer in 1951).  Though over time the HeLa cell line has lost many of its characteristics (such as some of the original chromosomes) over the decades, it is still considered a semi-standard for most tissue culture research.