Viruses are non-cellular infectious agents consisting of a single type of nucleic acid (either RNA or DNA) surrounded by a protein coat.  Viruses lack independent metabolism and therefore must reproduce only within living host cells.  Because viruses cannot reproduce independently, they are considered non-living (i.e. non-cellular).   Viruses are generally specific for a given host, but all organisms (including other microbes) are susceptible to viral attack.

As indicated above, most viruses consist of nothing more than nucleic acid surrounded by a protein shell - together these components represent one infectious particle, a virion.  The outer protein is refereed to as the capsid, which is made up of repeating protein subunits called capsomeres.   The architectural arrangement of the capsid determines the viruses general form, classified as being either polyhedral, helical, or binal.  

Other viruses, particularly those with animal hosts, may have a slightly more complex arrangements (see diagram below).  The most notable addition is a membranous envelope that surrounds the capsid.  Viruses that contain an envelope are termed "enveloped" whereas those that lack an envelope are called "naked".  The envelope is connected to the capsid by a layer of matrix proteins.  These proteins also serve to strengthen the envelope.  The envelope is comprised of glycoproteins (a protein-carbohydrate complex) embedded within lipid from the host's cell membrane.  Glycoproteins may also project from the envelope's surface to form spikes of varying lengths and shapes.  These spikes serve an important role in attachment and infection.  Because the origins of each component determines the virus' resistance and survival,  it is important to distinguish between components that are coded by the virus' genes (genes are segment of RNA or DNA) versus those that are coded by the host's genes.  Only the proteins of the viral envelope are coded by the virus.  The carbohydrate and lipids are coded by the host cell.  Therefore, the type of lipid and carbohydrate within a given viral envelope is dependent on the particular host.    

The fact that viruses lack their own means of replication distinguishes them from living microorganisms.  Below are some additional comparisons:

Viral Taxonomy (classification)

Animal viruses (viruses that require animal hosts) are classified based on:

• size

• the nature of the nucleic acid: DNA or RNA, single stranded or double stranded, segmented or single molecule

• the type of viral replication: type of nucleic acid, type of participating enzymes, location of replication within the host cell

• capsid structure: either polyhedral, helical, or binal 

• presence or absence of viral envelope

A more informal scheme involves classification based on routes of transmission:

How do viruses cause disease?

Viruses cause a wide variety of diseases in humans and all other living organisms.   But to understand how viruses causes disease, one must first know the basics of viral replication (i.e. reproduction involving the host's cellular mechanisms).  There are essentially 6 steps to replication, the details of which are varied and complex for a given type of virus.  The type of disease and its timing are influenced by each stage of the replication cycle. 

1. Attachment (adsorption) - the virion attaches to the host cell

2. Penetration - the virion breaks through the cell wall and/or membrane

3. Uncoating - the virion releases its nucleic acid from the protein coat

4. Replication - the virion exploits the host's cellular mechanisms for producing in order to produce more viral nucleic acid and viral proteins

5. Assembly (maturation) - viral nucleic acids and proteins are reassembled

6. Release - virions are released from the host cell; this generally results in cell and tissue damage and further activation of the immune system - together the manifestations of this process (as well as its location within the host) determine the type and extent of disease.

Some viruses result in Acute Infections in which the virions usually remain localized and the symptoms are typically short-lived.  The infected host cells will die upon release of new virions, resulting in cell and tissue damage.  Most likely, the host's defense mechanisms will eliminate the virus over a period of several  days or months.   Acute infections often result in the host organism's immunity to future infection.  Examples of acute infections include measles, mumps, and influenza (the flu).  

Other viruses cause Persistent Infections whereby virions persist within the host organism without any disease symptoms.  There are several different categories of persistent infections - the manifestations of which are influenced by the process of replication as outlined above.  Often persistent infections are actually the result of an acute infection.  For example, the measles exhibits relatively short infectious and disease phases, yet late complications in one in 300,000 individuals results in a fatal brain disorder (Subacute Sclerosing Panecephalitis (SSPE).  The new condition is the result of a defective viral  replication with brain cells.  When present, SSPE usually occurs within 10 years of having the measles. 

Other persistent infections are termed "latent" because they exhibit a long non-infectious stage between the original disease and the subsequent disease.  The best example of a latent infection is chicken pox and the shingles, both of which are caused by the same virus: Varicella-zoster virus.  Shingles or "zoster" appears after the virus becomes reactivated by unknown circumstances.  The herpes virus (both herpes simplex type 1 and 2)  is another example of a latent virus.

Persistent infections may also result in a continuous "chronic" infectious stages following a relatively brief disease stage.  Examples of chronic infections include hepatitis B and C.  Initial symptoms may include nausea, fever, and jaundice; however, the patient typically recovers from these early symptoms but remains infectious.  Over time the disease slowly manifests as hepatitis, cirrhosis of the liver, or cancer.  somewhat the opposite of a chronic infection is a "slow infection" in which no disease symptoms is initially observed but yet the infected person becomes more infectious over time.  Ultimately a disease becomes apparent.  A good example of a slow infection is AIDS, which is caused by the HIV virus.