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We usually put viruses into the same mental category as bacteria - a category that we popularly call "germs". We think of all germs as being somewhat alike, because we picture them as microscopic organisms that cause diseases.
We try to avoid exposure to germs, and we rely on vaccines and medicines for protection when we are exposed.
But this simple concept overlooks nearly everything of interest about viruses and bacteria. In particular, it ignores the fact that viruses and bacteria are completely different things - as different as night and day.
One huge difference is
that bacteria are living creatures, while viruses are not.
In either case, the
microscopic things enter our bodies and multiply rapidly. If a virus
is not a living creature, then how could it do this? Why are
bacteria considered to be alive, while viruses are not?
First of all, bacteria are cells. Every bacterium consists of a single, complete, living cell.
We sometimes refer to the contents of the cell - everything that is inside the plasma membrane - as protoplasm.
However, this generic term does not tell us much. In fact, the contents of the cell consist of a huge number of different materials - proteins, fats, carbohydrates, DNA, water, and so on.
A wide range of
biochemical processes go on within the cell at nearly all times, and
it is these ongoing processes - all operating under the
indirect control of the cell's DNA - that cause the cell to be
For example, temperatures that are too hot or too cold might kill it. Toxic compounds might kill it. Ripping open the plasma membrane will kill it. And because the cell is alive, it can also be starved to death.
The biochemical processes
that operate within the cell require energy. If the cell runs out of
energy - and if the cell cannot replace the energy by consuming an
appropriate food - then the cell will die.
Therefore they stand
alone in modern biological classification systems as the simplest
type of living organisms. (For more information on the
classification of living things, see article "How
Many Kinds of Living Things Are There?")
Food, water, and other
nutrients must be able to pass through the membrane into the
bacterial cell, and waste products must be able to exit the cell.
Just as a bacterial cell is much smaller and simpler than a human cell, a virus is much smaller and simpler than a bacterium.
A virus has no
protoplasm. It has no plasma membrane. It has no ongoing metabolic
processes. It does not consume food. It does not expel waste. It
cannot starve. Unlike a bacterium, a virus cannot reproduce on its
own. In fact, until a virus bumps into an appropriate host cell, it
remains a completely inert particle, without any of the essential
features we associate with living things - except for that
fragment of DNA or RNA.
RNA contains the same information as DNA, but in a normal cell the permanent copy of the genetic code is stored as DNA, while RNA is generally used to make temporary working copies of parts of that code.
Therefore viruses are not just random bits of genetic material - they are bits of genetic material that are capable of hijacking living cells.
The virus does not need to contain all the genetic information necessary to run the hijacked cell. It only needs enough DNA or RNA to redirect the activities of the cell. This can be compared to modern-day pirates who hijack an oil tanker.
The pirates might arrive next to the giant tanker in a tiny boat, even a rubber dinghy.
Once aboard the oil tanker, the hijackers don't need to know all the details of how to run the ship - they simply need to coerce the captain and crew to follow their orders.
Likewise, the DNA or RNA
in a virus takes over the hijacked cell, but some of the cell's
original DNA might still be needed in order for the cell to continue
In other viral diseases
the new viruses can escape by budding off while the cell continues
to manufacture more virus particles.
Because it is alive, it consumes energy, and therefore it requires food energy to stay alive. Bacteria have evolved countless different ways to make a living - that is, to get food - and only a tiny fraction of bacterial species cause disease.
A healthy bacterium can reproduce on its own by simply dividing itself into two parts. Viruses, on the other hand, are not alive - they are in essence just rogue bits of DNA or RNA. Until it hijacks a suitable host cell, a virus particle is completely inert.
The only way for a virus
to reproduce - or to do anything at all - is to hijack a living
cell from a susceptible species.
There are many types of viruses that are completely harmless to humans, even though they harm certain other species.
So how should we consider a virus that attacks a bacterium that causes human illness? The presence of the virus could actually help protect us from the bacterial disease.
You might say that the enemy of my enemy is my friend. The same goes for any other virus that attacks pests - or for that matter, any bacterium that attacks pests.
For example, we use the
Bacillus thuringiensis to help
control various insect pests that attack our crop plants.
This is especially true when we speak of "disease-causing microorganisms", also called pathogens.
The word "pathogen" originally meant anything that caused a disease, but now the term is usually limited to microorganisms. Besides viruses and bacteria, human diseases can also be cause by several other categories of infectious agents, although we often refer to most of these as parasites rather than pathogens or "germs".
For example, malaria,
which causes nearly a million deaths a year, is caused by a
microscopic protozoan rather than a virus or a bacterium.
Therefore it could be argued that there is no harm or confusion to speak of killing a virus.
However, such terminology can lead to misunderstandings. It is much more difficult to find drugs that are effective against viruses, compared to inventing anti-bacterial drugs (called antibiotics), for the precise reason that virus particles are not alive.
This greatly narrows the options for attacking the virus. It should also be noted that in most cases, antibiotics - which are designed to attack bacteria - are completely ineffective against viral diseases.
Therefore you cannot cure
a common cold by taking an antibiotic.
Well-known viral diseases include the,
Well-known bacterial diseases include,
In recent years, science has made significant progress in finding drugs to treat certain kinds of viral diseases.
However, because viral diseases are often so difficult to cure, most of the past emphasis has been on vaccines - which cannot cure the disease, but can help prevent the disease from occurring.
Vaccines work by training
the body to recognize and attack specific types of viruses at the
earliest stage of infection, before the viral infection gets out of
Therefore, any widely used antibiotic tends to have a limited useful lifetime, measured in decades - often shorter than the lifetime of a typical human being.
Another issue with antibiotics is that they often kill the "good" bacteria along with the harmful bacteria.
There are many, many
kinds of bacteria, most of which do not cause disease. In fact, many
bacteria are quite helpful to us, especially some of the bacteria
that live in the human intestines.
For most practical purposes, it makes sense to lump viruses with pathogenic bacteria when discussing ways to avoid and treat diseases spread by microorganisms.
However, because viruses
are not truly alive - and also because they are extraordinarily
small - we face additional hurdles when attempting to control
diseases that are caused by viruses, in comparison to diseases that
are caused by bacteria.