Some viruses do not kill host cells but rather exist within them in one form or another.
For example, certain of the viruses that can transform cells into a cancerous state are retroviruses;
their genetic material is RNA but they carry an enzyme that can copy the RNA's information into DNA
molecules, which then can integrate into the genetic apparatus of the host cell and reside there,
generating corresponding products via host cell machinery. Similarly, in bacterial DNA viruses known as
temperate phages, the viral nucleic acid becomes integrated into the host cell chromosomal material, a
condition known as lysogeny; lysogenic phages are similar in many ways to genetic particles in bacterial
cells called esipomes.
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