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e-Medical
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Viruses are the smallest of
parasites; they are completely dependent on cells (bacterial, plant, or
animal) to reproduce. Viruses are composed of an outer cover of protein
and sometimes lipid, and a nucleic acid core of RNA or DNA. In many
cases, this core penetrates susceptible cells and initiates the
infection.
Viruses range from 0.02 to 0.3 µ
[microns], too small for light microscopy but visible using electron
microscopy. Viruses can be identified by biophysical and biochemical
methods. Like most other parasites, viruses stimulate host antibody
production.
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Several hundred different viruses
infect humans. Because many have been only recently recognized, their
clinical effects are not fully understood. Many viruses infect hosts
without producing symptoms; nevertheless, because of their wide and
sometimes universal prevalence, they create important medical and public
health problems.
Viruses that primarily infect
humans are spread mainly via respiratory and enteric excretions. These
viruses are found worldwide but their spread is limited by inborn
resistance, prior immunizing infections or vaccines, sanitary and other
public health control measures, and prophylactic antiviral drugs.
Zoonotic viruses
pursue their biologic cycles chiefly in animals; humans are secondary or
accidental hosts. These viruses are limited to areas and environments
able to support their nonhuman natural cycles of infection (vertebrates
or arthropods or both).
Some viruses have oncogenic
properties: Human T-cell
lymphotropic virus type 1 (a retrovirus) is associated with human
leukemia and lymphoma. Epstein-Barr virus [EBV] has been associated with
malignancies such as nasopharyngeal carcinoma,
Burkitt's lymphoma, Hodgkin's disease and lymphomas in immunosuppressed
organ transplant recipients. Kaposi's sarcoma-associated virus is
associated with Kaposi's sarcoma,
primary effusion lymphomas and Castleman's disease (a
lymphoproliferative disorder).
Slow viral diseases
are characterized by prolonged incubations and cause some chronic
degenerative diseases, including subacute sclerosing panencephalitis
(measles virus), progressive rubella panencephalitis, progressive
multifocal leukoencephalopathy (JC virus) and Creutzfeldt-Jakob disease
(a prion disease). (See Slow
Virus Infections, Subacute Sclerosing Panencephalitis and Progressive
Rubella Panencephalitis.
Latency
- a quiescent infection by a virus - permits recurrent infection despite
immune responses and facilitates person-to-person spread. Herpesviruses
exhibit latency.
Diagnosis
Only a few viral diseases can be
diagnosed clinically or epidemiologically (eg, by well-known viral
syndromes: Measles,
Roseola Infantum, Erythema Infectiosum and Chickenpox).
Diagnosis usually requires testing. Serologic examination during acute
and convalescent stages is sensitive and specific but slow; more rapid
diagnosis can sometimes be made using culture, PCR [Polymerase Chain
Reaction] or viral antigen tests. Histopathology can sometimes be
helpful.
Prophylaxis and Treatment
Effective vaccines in general use
for active immunity include influenza, measles, mumps, poliomyelitis,
rabies, rubella, hepatitis A, hepatitis B, varicella, and yellow fever.
An effective adenovirus vaccine is available but is used only in
high-risk groups such as military recruits. Immune globulins are also
available for passive immune prophylaxis.
Effective therapeutic or
prophylactic drugs against some viruses are available: amantadine or
rimantadine for influenza A; acyclovir, valacyclovir, and famciclovir
for herpes simplex or varicella-zoster infections; ganciclovir,
foscarnet, and cidofovir for cytomegalovirus infections; and reverse
transcriptase inhibitors, protease inhibitors, and others for HIV.
Currently, the use of interferon is limited to hepatitis B and C and
human papillomavirus.
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