In a permissive host, the virus may cause death and lysis, but in a nonpermissive host, it may cause transformation of the infected host cell to a tumor cell. Most double-stranded DNA animal viruses replicate in the nucleus, although their replication strategies can be quite different. However, the pox viruses replicate in the cytoplasm using enzymes carried in the virion. Some of these viruses are now being used in genetic engineering experiments. Retroviruses are enveloped viruses that have complex life cycles since they are RNA viruses that replicate by means of a DNA intermediate.
Important retroviruses can cause cancer and acquired immunodeficiency syndrome.
The DNA becomes integrated into the host chromosome in the manner of a temperate virus. They are the smallest known pathogens. Unlike viruses, their extracellular form is the same as their intracellular form, and they have no protein coat. Prions have an extracellular form that does contain protein, but it does not contain the nucleic acid that encodes the protein.