What is a Virus – Definition

Virus – an infectious pathogen that must invade a host cell to replicate, technically called an obligate intracellular parasite. A virus is a particle of living material that contains an inner core of nucleic acid (DNA or RNA), called the genome, encased in an outer shell of protein, called a capsid.

Some viruses contain a third layer composed of lipids, called an envelope, that further protects and nourishes the virus. These components, collectively called a virion, cannot themselves support a full life cycle, which obligates the virus to find a host to maintain its survival.

A virus can attach only to the type of cell capable of supporting it, binding to specific protein molecules on the surface of the cell membrane.

Antibiotic medications are not effective in treating illnesses that result from viral infections, such as colds and influenza.

Common Viruses and The Illnesses they Cause

Virus or Viral FamilyGenetic ConfigurationIllness
adenovirusDNApharyngitis, pneumonia, acute respiratory disease, cervicitis, urethritis, cystitisgastroenteritis
cytomegalovirus (cmv)DNAcmv infection
epstein-barr virusDNAinfectious mononucleosis, Burkitt’s lymphoma, Hodgkin’s lymphoma
hepatitis A virus (HAV), hepatitis C virus (HBV)RNAhepatitis
herpes simplexvirus 1 (HSV-1)DNAcold sore
herpes simplex virus 2 (HSV-2)DNAgenital herpes
human herpesvirus 8 (HHV-8)DNAkaposi’s sarcoma
human immunodeficiency virus 1 (HIV-1),
human immunodeficiency virus 2 (HIV-2)
RNA retrovirusaids
human papillomavirus (HPV)DNAgenital warts, cervical cancer, vaginal cancer
human parainfluenza virusesRNAacute upper respiratory disease, CROUP, bronchiolitis, bronchitis, pneumonia
influenza virusesRNAinfluenza (flu)
measles virusRNAmeasles
mumps virusRNAmumps
Norwalk-like virusesRNAacute gastroenteritis
rabies virusRNArabies
respiratory syncytial virusRNAbronchiolitis, pneumonia, acute upper respiratory disease
rubella virusRNArubella (German or three-day measles)
varicella-zosterDNAchickenpoxherpes zoster(shingles)

How Viruses Work

After invading a host cell, a virus hijacks the cell’s structures and functions to serve its own needs and to replicate itself. DNA viruses produce proteins that the host cell’s RNA transcribe as instructions to replicate the virus’s DNA, which the cell does. DNA contains the instructions for the cell’s functions; RNA forms the messenger proteins that carry out the directives of the DNA.

Eventually the virus’s copies of DNA crowd out the cell’s copies of DNA, and the cell becomes the agent of the virus. The cell either divides or ruptures, spreading the virus. RNA viruses achieve a similar result by causing the host cell to replicate their RNA, which then replaces the cell’s RNA.

Retroviruses are RNA viruses that contain the enzyme reverse transcriptase, which allows RNA to instruct DNA (the reverse of normal).

Virus Adaptability

Viruses are highly adaptable and have numerous mechanisms to hide from the immune system, allowing them to become well established infections before the immune system detects their presence. Once the immune system does detect a virus, it develops antibodies that protect against subsequent infection by the same virus.

Many common viruses-such as those that are responsible for colds (rhinoviruses), gastroenteritis (enteroviruses), and the flu (influenza viruses)- frequently alter their structures, evolving into different strains that can cause the same illnesses.

Some viruses, such as human T-lymphotropic virus (HTLV) and human papillomavirus (hpv), cause cancer (oncoviruses). The human immunodeficiency virus (HIV) is a retrovirus that attacks the immune system, causing AIDS (acquired immunodeficiency syndrome).


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