PRINCIPLES OF VIROLOGY
Viruses are obligate intracellular parasites. At minimum viruses contains a viral genome (of either RNA or DNA) and a protein coat (the capsid). The genome can be either double stranded (ds) or single stranded (ss). The genome and its protein coat is a virion. Some viruses also have a phospholipid envelope surrounding the virion. Enveloped viruses are often more susceptible to inactivation by temperature, pH and chemicals than nonenveloped viruses. The envelopes are of host origin, but they contain virus-encoded proteins. The viruses acquire the lipid membranes as they bud off host cells.
The morphology of virions is either helical, icosahedral (a geometric shape with 20 faces), or complex. The envelope masks the shape of the virion, so most enveloped viruses are pleomorphic or variably shaped. The poxviruses are the largest viruses (250 by 350 nm) and the smallest human virus is the poliovirus, which is 25 nm in diameter.
Viruses are classified in families and genera based on genome types (RNA or DNA), the number of strands in the genome (ds or ss), morphology, and presence or absence of an envelope. International Committee on Taxonomy of Viruses classifies viruses into 71 families, 9 subfamilies, 164 genera, and 3600 species.
Viral reproduction, or replication, is unique to viruses. The virus attaches to the surface of a susceptible cell by means of specialized structures on its surface of the cells. The virus enters the cells by endocytosis, fusion of the viral membrane and cell membrane, or lysis of the cell’s membrane. Once inside the cell, the virus loses its coat, removing the capsid from the genome then directs the host cell to make viral proteins and genome. Depending on the virus, the metabolism of the host cell may be shut off completely or it may continue on restricted scale. The virus encoded proteins and genome then reassemble in the host cell. The new virions are then released by lysis of the infected cell or by budding through the cell membrane.
Clinically significant DNA viruses:
|dsDNA||Yes||Herpesviridae||Alphaherpesvirinae||Simplexvirus||Herpes simplex viruses|
|Varicellovirus||Varicella zoster virus|
|Gammaherpesvirinae||Lymphocryptovirus||Human herpesvirus 4|
|Molluscipoxvirus||Molluscum contagiosum virus|
|ss/dsDNA||Yes||Hepadnaviridae||Orthohepadnavirus||Hepatitis B virus|
Clinically significant RNA viruses:
|ssRNA||Flaviviridae||Yellow fever virus group||Yellow fever virus|
|Dengue virus group||Dengue virus|
|Japanese encephalitis group||Japanese encephalitis virus|
|Hepatitis C virus||Hepatitis C virus|
|Coronaviridae||Coronavirus||Human corona virus|
|Togaviridae||Alphavirus||Eastern/Western encephalitis virus|
|Retroviridae||Human T-cell leukemia retrovirus||Human T-cell leukemia virus|
|Picornaviridae||Enterovirus||Human polio virus|
|Hepatovirus||Hepatitis A virus|
|Deltavirus||Hepatitis D virus|