Viruses may be the most bizarre of all life forms,  although they are not truly living.  They are however a form of life and consist of mere short pieces of infectious deoxyribonucleic acid ( DNA) or ribonucleic acid (RNA) wrapped in a simple protective coat.  The famous immunologist, Sir Peter Medawar termed the virus "as a piece of bad news wrapped up in protein."   Of course, there is more than just the structure  (a virion) to consider, there is an outer coat called the capsid.  Capsids come in various sizes and shapes, each characteristic of the virus family to which it belongs.  They are built up of protein subunits called capsomeres and it is the arrangement of these around the central genetic material that determines the shape of the virion.  Most viruses are too small to be seen under a light microscope as they are about 100 to 500 times smaller than bacteria, varying in size from 20 to 300 nanometers in diameter (one nanometer is a thousand millionth of a meter).  Inside the virus capsid is its genetic material, or genome, which is either DNA or RNA depending on the type of virus. The genome contains the virus's genes, which carry the code for making new viruses, and transmits these inherited characteristics to the next generation.  Viruses usually have between 4 and 200 genes.

According to Frank Ryan MD, in his book Virus X," viruses have no skin, nerves, and for that matter, no brain, but they do have a way of detecting the chemical composition of targeted cell surfaces."  Every virus has a chosen host cell, whether it is the leaf of a tobacco plant in mosaic disease or the T cell in a human sufferer of AIDS.  The virus has the most unique ability to sense the right cell surfaces and recognizes them through a perception of three dimensional surface chemistry.  They then hijack the targeted cell's components and use what they need, often killing the cell in the process.  Thus viruses are obliged to obtain the essential parts of other living things to complete their life cycle. Inside the cell the virus reproduces.  The cell eventually bursts open, releasing the virus and the vicious cycle begins anew.

Recent research indicates that many of the viruses infecting us today have ancient evolutionary histories that date back to the first vertebrates and perhaps the first animals in existence.  Researchers discovered 214 novel RNA viruses (where genomic material is RNA rather than DNA) in apparently healthy reptiles, amphibians, lungfish, ray-finned and other types of fish. For the first time there is proof that RNA viruses are many million years old, and have been in existence since the first vertebrates existed.  The study emphasized how large the universe of viruses really is.  Viruses are everywhere and many millions are still to be discovered.  It has been estimated that trillions of viruses fall from the sky each day.  According to a recent study some 800 million viruses cascade onto every square meter of the planet swept into the air by sea spray, and dust storms.  It is assumed that these viruses originate on the planet and swept upward, but it is theorized that viruses may even have come from outer space.  Viruses are more than infectious agents, they are essential to everything from our immune system to our gut microbiome, to the ecosystems on land and sea, to climate regulation and the evolution of all species.   These surprising life forms contain a vast diverse array of unknown genes and spread them to other species;  they are far from simple.

Viruses are associated with plagues--epidemics accompanied by great mortality, such as AIDS, flu and smallpox. The latter may be the most fearsome virus.  It was believed to have originated in India in ancient times before first ravaging the Roman world as early as A.D. 165, since then it had scourged humanity in what amounted to a permanent pandemic, causing incalculable loss of life and misery through its morbidity and disfigurement.  In its more virulent form caused by the virus Variola major, it still caused up to a 50 percent mortality in its victims. As late as 1958, when Russian doctors pressed for a concerted world campaign against it through the World Health Organization, 2 million people still died from its effects each year.  A global campaign against smallpox began in 1967 which involved vaccinating as many as 250 million people yearly.  It took ten years to achieve success.  Physicians at that time felt it possible to eradicate all viral plagues.  Today such optimism appears unwarranted as new viruses such as  Chikungunya, Lassa, Nipah, or Zika seem to appear almost daily.