A virus can remain in equilibrium with the host and not actually
produce disease for a long period, often many years. The oncogenic
viruses just discussed are examples of such latent infections.
All of the human herpesviruses can remain in host cells throughout
the life of an individual. When herpesviruses are reactivated
by immunosuppression (for example, AIDS), the resulting infection
may be fatal. The classic example of such a latent infection
in viruses is the infection of the skin by herpes simplex virus,
which produces cold sores. This virus inhabits the host's nerve cells but causes no damage until it is activated by a stimulus such
as fever or sunburn- hence the term fever blister.
In some individuals, viruses are produced, but symptoms
never appear. Even though a large percentage of the human
population carries the herpes simplex virus, only iO to 15% of
people carrying the virus exhibit the disease. The virus of some
latent infections can exist in a lysogenic state within host cells.
The chickenpox virus (Varicellovirus ) can also exist in a latent
state. Chickenpox (varicella ) is a skin disease that is usually
acquired in childhood. The virus gains access to the skin via the
blood. From the blood, some viruses may enter nerves, where
they remain latent. Later, changes in the immune ( I -cell )
response can activate these latent viruses, causing shingles
(zoster). The shingles rash appears on the skin along the nerve in
which the virus was latent. Shingles occurs in 10 to 20% of people
who have had chickenpox.
Persistent Viral Infections:
A persistent or chronic viral infection occurs gradually over a
long period . Typically, persistent viral infections are fatal.
A number of persistent viral infections have in fact been
shown to be caused by conventional viruses. For example, several
years after causing measles, the measles virus can be
responsible for a rare form of encephalitis called subacute sclerosing
panencephalitis (SSPE). A persistent viral infection is
apparently different from a latent viral infection in that, in
most persistent viral infections, detectable infectious virus
gradually builds up over a long period, rather than appearing suddenly.
Cited By Kamal Singh Khadka
Msc Microbiology, NIST, TU, Kathmandu , Nepal