Obligate intracellular parasites that depend in the host’s metabolism for their replication.

Consist of a nucleic acid genome surrounded by a protein capsid that may be encased by a lipid membrane.

Can only replicate inside appropriate host cell.

The viral genome composed of a nucleic acid sequence made up of RNA or DNA.

Variation in genomic structures include: Double stranded, Single stranded. Segmented, Nonsegmented, Linear, circular or helical.

With a single stranded genome a virus may be Positive stranded-positive sense RNA, Negative stranded-negative sense RNA, or Ambisense containing both positive and negative strand.

After entry of the virus into cells via endocytosis, it is localized in acidic vacuoles; the M2 channel functions in transporting protons with the gradient from the vacuolar space into the interior of the virion.

Acidification of the interior results in disassociation of ribonucleoproteins, and the initiation of viral replication.

The capsid or capsomere forms an outer shell holding and protecting the viral genome and virus encoded enzymes.

The capsid is composed of proteins and serves as an antigenic stimulus for antibody formation.

Capsid contains receptor specific sites needed for initiating infection.

The capsid has several shapes including: Icosahedral, Helical and Complex.

Enveloped viruses refer to those surrounded by a lipoprotein membrane that is acquired on the nonlytic release of the virus from the cell.

For the virus to attach to the cell and initiate infection the virus envelope is encoded with glycoprotein throughout its structure.

Glycoproteins in the envelope also provide as viral antigens and stimulate antibody production.

DNA viruses usually acquire envelopes from the host’s nuclear membrane as the virus exit the nucleus.

Poxviruses are viruses which acquire their envelope in the cytoplasm unlike other DNA viruses.

Viral glycoproteins in the envelope serve as viral attachment proteins that bind to host cell structures and include: hemagglutinins which bind to erythrocytes, neuraminidase that facilitate release of virus from host cells, and fusion proteins that facilitate fusion of viruses and the host cell.

Enveloped viruses can be damaged by dehydration, organic solvents, detergents, and extreme temperatures or pH levels.

Classified by nucleic acid genome, shape of its capsid, presence of a lipid envelope, mode of replication, preferred cell type of replication, or the type of pathology it causes.

Viruses without envelopes, naked viruses, are more stable than enveloped viruses as they are better able to withstand injury from damaging environmental effects.

Three families of naked DNA viruses exist: Parvoviridea, Adenoviridae, and Papovaviridae.

Four families of naked RNA viruses exist: Astroviridae, Reoviridae, Picornaviridae, and Calciviridae.

Range from 20-300 nm in size.

May form inclusion bodies in cells which can be seen in CMV, rabies, smallpox and herpes virus infections.

Causes many human infections.

Associated with transient illness such as colds and influenza.

May cause infections where the virus is not eliminated from the body and persist within the host for years in a latent form or still multiplying.

Herpes zoster virus, the cause of chickenpox, can enter the dorsal root ganglia and establish latency to reactivate and cause shingles at a later time.

Some viruses can cause transformation of a host cell to a cancer cell, e.g. human papillomavirus implicated in cervical cancer.

Associated with 10 to 15% of all human cancers globally.

Seven viruses linked with human cancers including: Epstein Barr virus, human papilloma virus, hepatitis B virus, hepatitis C virus, human herpesvirus 8, human T cel l lymphotropic virus, and Merkel cell Polyomavirus.

The four infectious agents that account for approximately 13% of cancers are: H.pylori, HPV, and hepatitis B and C viruses.

Transmission may occur with direct contact with bodily fluids, via respiratory droplets, blood, mucus, saliva and semen.

Can be transmitted by injections and by organ transplants.

The earth is a global, human dominated ecosystem that allows for the emergence and host switching of animal viruses, especially genetically error prone RNA viruses.


RNA viruses have high mutation rates: it took the genome of the human species 8 million years to evolve by 1% and an animal virus can evolve by more than 1% in a matter of days.

Environmental factors such as temperature, humidity, and ventilation play a role in the persistence, infectivity, and dispersal and removal of viruses. 

Environmental factors can alter human defense mechanism that protects against respiratory pathogens.

When a virus infects a cell, it corrupts the cell’s protein-making machinery to crank out more copies of itself. 

When the cell dies, these new copies are released into the plasma outside the cell where they can float over and infect new cells. 

It is in the extracellular space where a virus can also be attacked by the neutralizing antibodies that the immune system makes to fight it off.


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