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To most people, it seems self-evident that a virus is some kind of living creature. 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.
This, of course, is a counter-intuitive idea. How in the world could a virus not be a living thing? We know that viruses — or at least some kinds of viruses — can make people sick, just as some kinds of bacteria can make us sick. 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?
Let’s first look at the characteristics of bacteria, to see why bacteria are considered to be alive. First of all, bacteria are cells. Every bacterium consists of a single, complete, living cell. On the outside is a porous cell wall, which provides rigidity and determines the shape of the bacterium. Inside of that is a plasma membrane, which is a thin layer that keeps the cell intact and separates the contents of the cell from the rest of the world. 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 “alive”.
Because a bacterium is alive, it can be killed. Anything that permanently interrupts the ongoing biochemical processes will kill the cell. 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.
Even though bacteria are one-celled creatures, their cells are smaller and simpler than human cells. In fact, bacterial cells are much simpler than the cells of all other living things — even those of other single-celled organisms. 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 my earlier article “How Many Kinds of Living Things Are There?”)
Even though the plasma membrane of a bacterium separates it from the outside world, the membrane must be selectively permeable in order for the bacterium to remain alive. (This attribute can be temporarily suspended in certain bacteria that go dormant in the form of highly durable endospores.) 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.
Viruses are quite different from bacteria. A virus is typically just a fragment of DNA or RNA wrapped in a protective protein coat. (Some viruses contain a bit more than this, but not much more.) 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.
(Note: 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.)
However, a virus does have two very important features that make it extremely powerful:
1) The protein coat can adhere to the cell membrane of an appropriate host cell, after which the viral DNA or RNA enters into the cell.
2) The invading DNA or RNA redirects the metabolic activities of the hijacked cell, turning the cell into a factory to crank out lots more virus particles.
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 operating.
With the machinery of the hijacked cell redirected to manufacturing more virus particles, the newly created viruses need a way to escape, in order to go infect other cells. In some viral diseases no viruses escape the host cell until the cell is packed full of new virus particles, at which time the cell ruptures — killing the cell but releasing a great quantity of the virus. In other viral diseases the new viruses can escape by budding off while the cell continues to manufacture more virus particles.
So now let’s review a few of the key differences between a bacterium and a virus. A bacterium is a tiny living creature consisting of a single, simple living cell. 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.
Because all viruses make their living by hijacking living cells, you might therefore say that all viruses are harmful. However, this ignores one key point. Each type of virus requires a particular host species — or a range of somewhat related host species — in order to infect and kidnap the corresponding cells. 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 bacterium Bacillus thuringiensis to help control various insect pests that attack our crop plants.
Despite the fact that viruses are not alive, and are not given a place in the taxonomy of living things, we often include viruses when we speak of “microorganisms”. Because all other organisms are alive, this inclusion is rather misleading — even though it makes sense in many other ways. 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.
If a virus is not alive, then does it make any sense to speak of “killing” a virus? Strictly speaking, a virus cannot be killed — but a virus can be destroyed, which for all practical purposes amounts to the same thing. 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.
So this raises the question: Which common diseases are caused by viruses, and which are caused by bacteria? Well-known viral diseases include the common cold, influenza (flu), chickenpox, HIV/Aids, herpes, mumps, measles, German measles (rubella), shingles, viral hepatitis (types A through E), zika, chikungunya, rabies, polio, West Nile, dengue, yellow fever, and ebola. Well-known bacterial diseases include cholera, tuberculosis, typhoid, tetanus, Lyme disease, chlamydia, salmonellosis, syphilis, diphtheria, leprosy, bubonic plague, pertussis (whooping cough), listeriosis, psittacosis, rheumatic fever, scarlet fever, anthrax, and strep throat.
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 control.
In contrast, we have a wide range of antibiotics that are (or used to be) effective against a wide range of bacterial diseases. Unfortunately, the more we use any specific antibiotic, the more likely we are to breed antibiotic-resistant strains of those diseases. 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.
In conclusion, a virus is not a living creature — and in fact, it is little more than a rogue piece of genetic material (DNA or RNA). And yet, because of its ability to commandeer the biological processes of a living cell, a virus acts much like a living creature after it hijacks the cell. 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.