Episomal<unk>  Episomal<unk>  refers to the use of genetic episomes during<unk> . In this<unk>  type, viral genes are stabilized, floating in the cytoplasm or nucleus as distinct objects, either as linear or lariat structures. Episomal<unk>  is more vulnerable to ribozymes or host foreign gene degradation than proviral<unk>  (see below). One example is herpes<unk>  family, Herpesviridae, all of which establish latent infection. Herpes<unk>  include chicken-pox<unk>  and herpes simplex<unk> es (HSV-1, HSV-2), all of which establish episomal<unk>  in neurons and leave linear genetic material floating in the cytoplasm. The Gammaherpesvirinae subfamily is associated with episomal<unk>  established in cells of the immune system, such as B-cells in the case of Epstein–Barr<unk> . Epstein–Barr<unk>  lytic reactivation (which can be due to chemotherapy or radiation) can result in genome instability and cancer. In the case of herpes simplex (HSV), the<unk>  has been shown to fuse with DNA in neurons, such as nerve ganglia or neurons, and HSV reactivates upon even minor chromatin loosening with stress, although the chromatin compacts (becomes latent) upon oxygen and nutrient deprivation. Cytomegalovirus (CMV) establishes<unk>  in myeloid progenitor cells, and is reactivated by inflammation. Immunosuppression and critical illness (sepsis in particular) often results in CMV reactivation. CMV reactivation is commonly seen in patients with severe colitis. Advantages of episomal<unk>  include the fact that the<unk>  may not need to enter the nucleus, and hence may avoid nuclear domain 10 (ND10) from activating interferon via that pathway. Disadvantages include more exposure to cellular defenses, leading to possible degradation of viral gene via cellular enzymes. Reactivation may be due to stress, UV, etc. Proviral<unk>  A provirus is a<unk>  genome that is integrated into the DNA of a host cell. Advantages include automatic host cell division results in replication of the<unk> 's genes, and the fact that it is nearly impossible to remove an integrated provirus from an infected cell without killing the cell. A disadvantage of this method is the need to enter the nucleus (and the need for packaging proteins that will allow for that). However,<unk> es that integrate into the host cell's genome can stay there as long as the cell lives. One of the best-studied<unk> es that does this is HIV. HIV uses reverse transcriptase to create a DNA copy of its RNA genome. HIV<unk>  allows the<unk>  to largely avoid the immune system. Like other<unk> es that go latent, it does not typically cause symptoms while latent. Unfortunately, HIV in proviral<unk>  is nearly impossible to target with antiretroviral drugs. Several classes of<unk>  reversing agents (LRAs) are under development for possible use in shock-and-kill strategies in which the latently infected cellular reservoirs would be reactivated (the shock) so that anti-viral treatment could take effect (the kill). Maintaining<unk>  Both proviral and episomal<unk>  may require maintenance for continued infection and fidelity of viral genes.<unk>  is generally maintained by viral genes expressed primarily during<unk> . Expression of these<unk> -associated genes may function to keep the viral genome from being digested by cellular ribozymes or being found out by the immune system. Certain viral gene products (RNA transcripts such as non-coding RNAs and proteins) may also inhibit apoptosis or induce cell growth and division to allow more copies of the infected cell to be produced. An example of such a gene product is the<unk>  associated transcripts (LAT) in herpes simplex<unk> , which interfere with apoptosis by downregulating a number of host factors, including major histocompatibility complex (MHC) and inhibiting the apoptotic pathway. A certain type of<unk>  could be ascribed to the endogenous retroviruses. These<unk> es have incorporated into the human genome in the distant past, and are now transmitted through reproduction. Generally these types of<unk> es have become highly evolved, and have lost the expression of many gene products. Some of the proteins expressed by these<unk> es have co-evolved with host cells to play important roles in normal processes.
Virus latency
Vaccine resistance
Criminal transmission of HIV
Spillover infection
Viral pathogenesis
Vaccine shedding
Subclinical infection
Drug resistance