The common public perception is that the only function of viruses is to cause disease, misery and death. This is incomplete and inaccurate. I will try to explain why. To understand most of it, unfortunately, I need to introduce some difficult concepts from microbiology, genetics and etiology. I hope that the average reader will understand most of them.
Viruses cause many common diseases that the average person is not aware of: cold sores, hepatitis B, herpes, yellow fever, viral meningitis, chicken pox, colds, mononucleosis, mumps, rabies, polio, shingles, smallpox, warts, viral pneumonia, AIDS and some cancers.
Defining a virus has been a challenge since the beginning of their discovery, because every time the scientists come up with a good definition someone discovers a virus that breaks the rules. The best thing that we have so far: Viruses are entities that infect cellular life. They are very diverse. The simplest viruses just have a couple of genes made of RNA or DNA wrapped up in a protein coat. Others have hundreds of genes, more than some bacteria. Based on the actual criteria of what a living been is and does, the viruses are not considered living entities. Viruses do not grow, metabolize or maintain a constant internal environment. So, by this definition, viruses are not “alive”.
Viruses are the ultimate freeloaders – they sneak into our cells, eat our food and rely on our homeostasis (their favorite temperature just happens to be body temperature!).
All viruses are ultimately parasites. They require a host for replication. They cannot generate their own energy like normal (human) cells can.
The name Virus comes from a Latin word meaning “slimy liquid” or “poison.” The earliest indications of the biological nature of viruses came from studies in 1892 by the Russian scientist Dmitry I. Ivanovsky and in 1898 by the Dutch scientist Martinus W. Beijerinck.
A virus is an infectious agent of small size and simple composition that can multiply only in living cells of animals, plants, or bacteria. They range in size from about 20 to 400 nanometres in diameter (1 nanometre = 10-9 meters) which is about 100 times smaller than the average bacteria and can only be observed by electron microscopy. By contrast, the smallest bacteria are about 400 nanometres in size.
The origin of viruses
There are three prevalent theories in the academia about the origin of viruses.
- Some scientists hypothesize that viruses ‘evolved’ from bacteria by natural selection. In this process, as they become parasites, they lost all the complex protein structures that bacteria require.
- Owing to their simplicity, other researchers support the theory that viruses were the first form of life, and that bacteria evolved from them (as did all other life). The huge problem with this theory is that viruses are not living being, and in order to reproduce and to make ATP, they require all of the complex cellular machinery present in bacterial cells.
- Other scientists speculate that a reverse symbiosis occurred, and that viruses arose out of cell parts such as bacterial plasmids and other organelles, and eventually evolved into separate forms of life.
So far evidence is lacking for each of these theories. To date, no clear explanation for the origin(s) of viruses exists.
What are some benefits of the viruses?
While the most familiar viruses cause disease in humans, there are viruses that infect organisms at every scale, from plants and animals all the way down to microbes. By preying on other creatures, viruses play a role in natural selection. This selection is of great importance on the microbial level because of the huge quantity of viruses that exist in the environment. I think this is the most important concept about viruses and their importance in the life on Earth. Please read on to better understand this.
Many plant viruses confer drought tolerance or cold tolerance to plants. We don’t always know how this works but, for example, elevated sugar is very common in virus-infected plants. More sugar would allow the plant cells to retain more water, protecting them from drought. And you know, things that are really sweet freeze slowly, so extra sugar would make plants cold-resistant.
And in animals, actually in mice, herpes viruses confer resistance against bubonic plague. That’s because the herpes virus, dormant in the mouse, turns up the mouse’s immune system and makes it better able to fight the plague.
Similarly, in people, hepatitis G virus may offer some protection against AIDS. Hepatitis G, now called pegivirus or GB virus C, is quite common in humans, and isn’t known to cause any disease. But it does affect the immune system in a variety of ways. If people are infected with hepatitis G first, and then HIV, it takes longer for it the HIV to progress to AIDS.
In the aquatic ecosystems that have been studied (the oceans), viruses have a greater abundance than bacteria by a ratio of roughly 10 to 1. There are more viruses than there are stars in the universe! Most of these ocean viruses infect bacteria. And, overall, viruses may outnumber all other biological forms on earth. Some of this viral “grazing” impacts systems that we can observe—for example one research group found that viruses contribute to the demise of algal blooms, growths of photosynthesizing organisms on the surfaces of seas or freshwaters that can span thousands of miles.
The Role of Viruses in Life
The importance of viruses is closely related to the importance of bacteria. We cannot discuss about the role of viruses if we don’t bring into the discussion the role of bacteria and the relationship between them and the humans. As Lynn Margulis once noted, microorganisms have long been considered ‘tiny little beings that are primarily germs and pathogens.’ In contrast to this public image, bacteria are at the basis of our life-support system. They supply our fertile soil and atmospheric gases. They cleanse our water supply, play a role in stabilizing the atmospheric nitrogen concentration, regulate the acidity or alkalinity of the soil environment, and thus generally ensure that our world is livable.
The view now emerging of the normal relationship between viruses and genes is not so much a host/invader relationship, but a relationship more akin to bees carrying pollen from flower to flower, thus causing cross-fertilization. Viruses carry not only their own genes, but also those of other creatures as well, especially those of bacteria. Although bacteria pass genetic information to each other using several processes such as pili transfer, viral transfer is now known to be critically important.
It is believed in some circles that viruses convert all bacteria into one giant, global ‘super-organism’, and that viruses possess a remarkable mechanism for the creation and exchange of genetic material. A major class of genes exchanged are antibiotic-resistance genes, along with genes that allow bacteria to degrade toxins (such as poly-chlorinated biphenyls) or convert mercury to less noxious forms. This ability is significant in the development of resistance to antibiotics produced by other organisms, allowing bacteria to survive and helping to maintain the balance so critical for ecology.
The traditional understanding that viruses are alien invaders competing against humans in a life or death struggle for the cell’s manufacturing facility is now understood to be oversimplified, if not incorrect. It is usually not expedient (smart) for a virus to kill its host, since this may cause the death of the virus. They are smart and want to live in you in a symbiotic relationship. Viruses must have then a reservoir of host species in which they can live permanently otherwise they would soon go extinct. AIDS, for example, infects some monkeys without causing illness or death, and has probably lived in them in a friendly relationship for generations. The host organism must tolerate them fairly well—in fact, some kinds of viruses form a symbiotic relationship with their hosts. One study examined meconium — a newborn’s first poop — and found no evidence of viruses. However, just 1 week after birth, each gram of a baby’s poop contained about 100 million virus particles, most of which were bacteriophages (“killing bacteria”). Our virome truly is a lifelong companion.
The perfect scenario for a virus is when its transmissibility is high and the mortality rate is low. The Perfect Storm.
Pathogenic viruses are only the ‘tip of the iceberg’ of virus types, and the more that is learned about the biological world the more scientists are coming to realize the critical role that viruses play in life. The fact that so few kinds of microorganisms are pathogenic is evidence for the mutation theory of the origins of harmful viruses. Another piece of evidence is the fact that usually most strains are not pathogenic, with only one strain or a few strains causing problems. Furthermore, often viruses do not kill directly, but indirectly by skewing the immune response of the person.
To recap in the end in the simplest form, while viruses can cause great harm, they also play a role that often goes unnoticed. Like a shallow stream rolling through a canyon or the wind that erodes rough stones, viruses shape the landscape of the microbial world and thus re-shape Life on Earth.