Скачать или смотреть How Virus Affect a Host Cell? | Conceptional Explanation

Many virus infections (e.g., influenza) are acute infections—that
is, they have a fairly rapid onset and last for a relatively short time. However, some viruses can establish persistent infections lasting many years. There are several kinds of persistent
infections. In chronic virus infections, the virus is almost always
detectable and clinical symptoms may be either mild or absent for
long periods. Examples are hepatitis B virus and HIV. In latent
virus infections the virus stops reproducing and remains dormant
for a period before becoming active again. During latency, no
symptoms or viruses are detectable, although antibodies to the
virus may be present at low levels. Examples are herpes simplex
virus, varicella-zoster virus, cytomegalovirus, and Epstein-Barr
virus, which causes mononucleosis. Herpes simplex type 1 virus
often infects children and then becomes dormant within the nervous system ganglia; years later it can be activated to cause cold
sores. The varicella-zoster virus causes chicken pox in children
and then, after years of inactivity, may produce the skin disease
shingles (initial adult infections result in chicken pox).
The causes of persistence and latency are probably multiple,
although the precise mechanisms are still unclear. The virus
genome may be integrated into the host genome thereby becoming a provirus. Viruses may become less antigenic and thus less
susceptible to attack by the immune system. Often virus infections are latent in a site not subject to immune attack, such as the
central nervous system. Viruses may also mutate to less virulent
and slower reproducing forms. Sometimes a deletion mutation
produces a defective interfering (DI) particle that cannot reproduce but slows normal virus reproduction, thereby reducing host
damage and establishing a chronic infection A small group of viruses causes extremely slowly developing
infections, often called slow virus diseases or slow infections, in
which symptoms may take years to emerge. Measles virus occasionally produces a slow infection. A child may have a normal
case of measles, then 5 to 12 years later develop a degenerative
brain disease called subacute sclerosing panencephalitis (SSPE).
Lentiviruses such as HIV also cause slow diseases.
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