What, Exactly, Is A Virus?

An illustration of the coronavirus. (Centers of Disease Control)

By now, the coronavirus is an all too familiar menace to most of the peoples of the world.  How it is spread,  the symptoms of the disease,  the absolute necessity of taking precautions against it — most people know something about the coronavirus pandemic.

But the question of what a virus actually is, what are its characteristics and where do they come from,  this seem to be far less well understood by the public.

So here is a primer on this often so destructive agent and its provenance — a look into the complicated, sometimes deadly and yes, fascinating world of viruses.

Viruses are microscopic pathogens that have genetic material (DNA or RNA molecules that encode the structure of the proteins by which the virus acts), that have a  a protein coat (which surrounds and protects the genetic material), and in some cases they have an outside envelope of lipids.

Most virus species have virions — the name given to a virus when it is not inside a host cell. They are too small to be seen with an optical microscope because they are one hundredth the size of most bacteria.

Transmission electron microscope image shows SARS-CoV-2, the virus that causes COVID-19, isolated from a patient in the U.S. Virus particles are emerging from the surface of cells cultured in the lab. The spikes on the outer edge of the virus particles give coronaviruses their name, crown-like. (NIAID-RML)

Unlike bacteria, viruses are generally not considered to be “alive.”

Although viruses do have genomes, they need to take over the machinery of other living cells to follow their own genome instructions.  This is why viruses cannot reproduce by themselves — as opposed to non-viral parasites  that can reproduce outside of a host cell.

Viruses are also too small and simple to collect and use energy, i.e., perform metabolism.   So they just take energy from the cells they infect, and use it only when they make copies of themselves.  They don’t need any energy at all when they are outside of a cell.

And viruses have no way to control their internal environment,  and so they do not maintain their own homeostasis as living creatures do.

These limitations are what lead many scientists to describe viruses as “almost alive,” which is a complicated state of existence indeed.

Infectious particles of an avian influenza virus emerge from a cell.  (Takeshi Noda/University of Tokyo)

Viruses are the most common form of life-like material on Earth — more common than bacteria and less well understood.  About 500 virus species have been described in some detail, but millions more exist.  They can infect animals, plants and micro-organisms, including bacteria and archaea.

Viruses infect in an endless number of ways.  They can be transmitted from plant to plant by insects that feed on plant sap, such as aphids;  and from animal to animal via blood-sucking insects.

Influenza viruses are spread by coughing and sneezing while norovirus and rotavirus — common causes of viral gastroenteritis — travel by the fecal–oral route, passed by contact and entering the body in food or water.

HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood.  And, of course, today’s coronavirus is a respiratory virus which spreads primarily through droplets produced when an infected person coughs or sneezes, or through droplets of saliva or discharge from the nose.  It remains active in the air for up to 30 minutes and on surfaces for up to several days (with diminishing potency.)

The variety of host cells that a virus can infect is called its “host range.” This can be narrow, meaning a virus is capable of infecting few species, or broad, meaning it is capable of infecting many.

The coronavirus wrecking havoc around the world today is believed to have originated in bats — having been transferred to another wild animal that was then sold and consumed and jumping in that way to humans.  In this animal-to-human progression, today’s coronavirus is similar to other recent coronaviruses such as Severe Acute Respiratory Syndrome (SARS) and Middle Eastern respiratory syndrome (MERS.)

The Centers for Disease Control says there have been no confirmed cases of transmitting the resulting COVID-19 virus between humans and pet animals, or from pets to humans.

Viruses have innumerable types of structures.

Wherever there is life there are viruses, which has led scientists to conclude they have most likely existed since the origin of the first living cells.  But no proof of this is likely to be found because viruses do not form fossils.

Some  molecular techniques are used to investigate how they arose.  The vertical transfer of viral genes into other organisms sometimes remains in the germline of the host organisms and can be passed on to the offspring of the host for up to millions of years (and studied by virologists.)

How viruses entered the world remains hotly debated.  The theories include:

• Viruses may have once been small parasite cells — which could reproduce and metabolize on their own — inside larger cells. Over time, genes not required because of their parasitism were lost. The bacteria rickettsia and chlamydia are living cells that, like viruses, can reproduce only inside host cells. They lend support to this “regressive” hypothesis, as their dependence on parasitism is likely to have caused the loss of genes that enabled them to survive outside a cell.
• Some viruses may have evolved from bits of DNA or RNA that “escaped” from the genes of a larger organism. The escaped DNA could have come from plasmids (pieces of naked DNA that can move between cells) or transposons (molecules of DNA that replicate and move around to different positions within the genes of the cell). Once called “jumping genes”, transposons are examples of mobile genetic elements and could be the origin of some viruses. This is sometimes called the “vagrancy hypothesis,” or the “escape hypothesis.”
• The “virus-first hypothesis” proposes that viruses may have evolved from complex molecules of protein and nucleic acid at the same time that cells first appeared on Earth and would have been dependent on cellular life for billions of years.

But there are problems with all of these hypotheses: the regressive hypothesis did not explain why even the smallest of cellular parasites do not resemble viruses in any way. The escape hypothesis did not explain the complex capsids and other structures on virus particles. And the virus-first hypothesis erases the very definition of viruses in that they require host cells.

Viruses are now recognized as ancient and as having origins that pre-date the divergence of life into the three domains of bacteria, archaea and multi-cell eurkaroytes with enclosed nuclei.

A comparison of the helix and base structure of RNA (less evolved) and DNA (more evolved.) The origin of viruses may well have occurred in the “RNA World” that many scientists hold existed before our more complex DNA world.

The evidence for an ancestral world of RNA cells and analysis of viral and host DNA sequences are giving a better understanding of the evolutionary relationships between different viruses and may help identify the ancestors of modern viruses.

It seems unlikely that all currently known viruses have a common ancestor and viruses have probably emerged numerous times in the past by a variety of theorized or not-yet-imagined mechanisms.

The recent major viral epidemics — the flus of 1918, 1957 and 1968; and SARS, MERS and Ebola –all jumped from animals to humans and all are caused by viruses that encode their genetic material in RNA, and not the far more complex and evolved DNA.