Virology – Basics of Medical 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 replication:

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: 

Genome Envelope Family Subfamily Genus Significant members
dsDNA Yes Herpesviridae Alphaherpesvirinae Simplexvirus Herpes simplex viruses
Varicellovirus Varicella zoster virus
Betaherpesvirinae Cytomegalovirus Cytomegalovirus
Gammaherpesvirinae Lymphocryptovirus Human herpesvirus 4
Poxviridae Chordopoxvirinae Orthopoxvirus Vaccinia
Molluscipoxvirus Molluscum contagiosum virus
No Adenovirinae Mastadenovirus Human adenovirus
Papovavirinae Papillomavirus Human papillomavirus
ssDNA No Parvoviridae Parvovirinae Erythrovirus B19
ss/dsDNA Yes Hepadnaviridae Orthohepadnavirus Hepatitis B virus


Clinically significant RNA viruses: 

Genome Envelope Family Genus Significant Members
dsRNA No Reoviridae Orthoreovirus Human reovirus
Rotavirus Human rotavirus
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
Rubivirus Rubella virus
Retroviridae Human T-cell leukemia retrovirus Human T-cell leukemia virus
Lentivirus HIV
No Caliciviridae Calicivirus Norwalk virus
Picornaviridae Enterovirus Human polio virus
Rhinovirus Human rhinovirus
Hepatovirus Hepatitis A virus
SsRNA Yes Orthomyxoviridae Influenzavirus Influenzae virus
Filoviridae Filovirus Ebola virus
Rhabdoviridae Lyssavirus Rabies virus
SsRNA Arenaviridae Arenavirus Arena virus
Deltavirus Hepatitis D virus
Bunyaviridae Hantavirus Hantavirus

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