Viral Disease

Introduction

Viruses are small particles, usually a few hundred nanometers in size, with minimal proteins and genetic material packaged inside a proteinaceous capsid material. The genetic material in viruses can be either RNA or DN A. Viruses need a host cell for their replication and depend on the cellular machinery of the host for replication and to cause a disease. Important viruses and viral diseases include arbovirus and renavirus, bunya viruses, cytomegalovirus, papillomavirus, Epstein-Barr virus, genital herpes, German measles (rubella), hanta viruses, hemorrhagic viruses, herpes simplex, human immunodeficiency disease, influenza A, Kaposi's sarcoma, measles (rubeola), poliomyelitis, poxvirus Infections, rabies, respiratory syncytial virus infection, Varicella-Zoster viral disease, viral encephalitis, viral hepatitis, and West Nile virus.

Definition

Viral diseases are caused by organisms that are particles and do not have a self-sustaining or autoreplicatory cellular machinery, and contain nucleic acid as the genetic material.

Classification

Viruses are classified as RNA viruses and DNA viruses, based on their genetic material. Each type of virus contains a minimal set of proteins and genetic material. The genetic material may be either single stranded (poliovirus or human immunodeficiency virus) or double-stranded (herpes simplex virus or hepatitis B virus). The majority of RNA viruses carry single stranded genome. Exceptions to this include influenza A, bunya viruses, rotavirus, arena virus, and other reoviruses. Most DNA viruses carry a double stranded genome, with an exception being parvoviridae Kok and Pechere (1999). Other classifications for viruses are based on the polarity of the genomes, type of nucleocapsid, or shape of the viral particle.

Consequences

Viruses infect host cells, impairing their functions. The human immune system is capable of mounting a defense by generating the production of interferons and antibodies against viral infections.

Associated Disorders

Associated disorders depend on the type of viral infection and its virulence. For example, human immunodeficiency virus, which impairs the immune system, results in the development of opportunistic infections by other organisms, including other viruses.

Etiology

Individual viral diseases are caused by exposure to a particular virus.

Epidemiology

Viral diseases occur worldwide. While certain viral diseases are more common in particular regions, such as Hepatitis A in locations with poor sanitation, and Hanta virus on the Korean peninsula, there is no geographic limit to these diseases. For example, while West Nile infections were originally found only in African countries, they have now appeared in North America. Severe Acute Respiratory Syndrome (SARS) viral infections, for which there is no treatment, was first encountered in China, but was ultimately detected in North America due to its ease of transmission.

Pathophysiology

Viral infections occur in stages, some of which are specific to a particular virus. Moreover, viruses such as human immunodeficiency and hepatitis C virus are latent in host for months to years. In general, after entering the host, viruses locate their target cell through recognition of the host cell surface receptors complementing the viral cell surface proteins. After fusion of the viral particle with the host cell membrane, and subsequent release of the viral contents into the cell, new viral particles are produced by the host cell machinery utilizing the viral-specific proteins/nucleic acids. Following assembly and packaging with viral capsid and envelope, the mature viral particles infect other host cells. These particles are able to evade the host immune system, and to develop resistance to antiviral medications through mutations that occur during the replication process.

Signs and Symptoms

Symptoms commonly associated with viral infections include fever and weakness. The quality and quantity of symptoms will vary with the type of viral infection.

Standard Therapies

Antiviral agents include amantadine and rimantidine, which are used for the treatment of the common cold (rhinoviral disease), and interferon for hepatitis C infection. There are no drugs that are efficacious against all viruses. Many viral infections are treated only symptomatically as the condition will usually remit on its own within a matter of days. Sometimes antibiotics are prescribed to treat secondary bacterial infections.

Other Information – Web Sites

The following websites provide useful information on viral diseases:

Centers for Disease Control website: http://www.cdc.gov

World Health Organization website: http://www.who.int/

University Library at Karolinska Institute website: http://www.mic.ki.se/Diseases/C02.html