By Charles Nicolson
This article about viruses was written in February this year before the Corona ID -19 virus which had started in China exploded into the current global pandemic.
In recent years nothing has grabbed the attention of the populace world-wide as much as the escalating mortality toll caused by the onslaught of this virus as it continued to spread. News media have concentrated intensely on reporting infections, rising numbers of deaths (and recoveries) and the dedication of courageous and heroic medical and health workers. However, understandably there has been virtually no information as yet regarding the basic question: what is a virus?
According to microbiologists, viruses are by far the most abundant biological entities on earth, outnumbering the total of all the others added together. They infect all types and sizes of cellular life including animals, plants bacteria and fungi. Viruses are considered by some microbiologists to be a life form because they contain and carry genetic material, reproduce, and undergo alterations and changes to their composition and characteristics along lines similar to the concept of natural selection, although they lack key characteristics such as cell structures that are generally considered necessary in any entity for it to be defined as a ‘life form’. Because they possess some but not all of these attributes and properties, viruses have been described as ‘organisms at the edge of life’.
In a previous Getting Technical article there was reference to a report in the Star newspaper that the existence of the smallest sub-nuclear particle, until then proposed in theory under the name of the ‘Higgs Boson’, had recently been proven in an experiment at the CERN complex in Europe. The mass of the Higgs Boson tends to zero so it is regarded as representing a state of transition between energy and mass or, in other words, ‘an entity at edge of mass’ which could be viewed as analogous to a virus being ‘at the edge of life’.
Viruses are smaller and simpler than bacteria. By themselves they are inert and as such are not alive. They are just wherever they are and not doing anything or reacting with any other entities or substances. However, when they come into contact with any suitable type of host they tend to ‘spring into life’ by hijacking genetic substances inside host cells and combining with these substances in ways which allow the viruses to reproduce themselves rapidly.
As already mentioned, viruses are small. In general, viruses are about one hundredth of the average size of bacteria, which measure out at approximately one micron. At a hundred times smaller, viruses are therefore approximately one hundredth of one millionth of a metre – which explains why viruses are not visible under conventional optical microscopes. It was not until the first electron microscopes became operational during the early 1940s that the first visual images of viruses were able to be observed.
Figure 1: Electron microphotograph of multiple bacteriophages attached
to the wall of a bacterial cell. Image credit: University of Pennsylvania
The caption on Figure 1 above refers to the viruses attached to the bacterial cell wall as ‘bacteriophages’. This is a typical example of technical nomenclature used in microbiology which can get very complex and confusing. Bacteriophages are types of viruses which almost exclusively attack only specific species of bacterial cells. To avoid using too many technical terms in this article, all viral types will come under the term ‘virus’.
Figure 1 also shows clearly how much smaller viruses generally are than bacteria which explains to some extent the different opinions on the effectiveness or otherwise of people getting infected when wearing face masks by breathing in the Corona Covid-19 or spreading the virus when exhaling. A further complication in assessing how effective face masks are is whether Covid-19 coming into contact with masks are just the viruses as such or the viruses contained in much larger droplets of fluid or saliva sprayed out by sneezing and coughing.
Different types of viruses can infect only a limited range of hosts and many are species-specific. Some, such as the smallpox virus for example, can infect only one species—in this case humans. Other viruses, such as rabies virus, can infect different species of mammals. The viruses that infect plants are harmless to animals and most viruses that infect animals are harmless to humans.
The host range of some particular viruses is limited to a single strain of bacteria which enables these viruses to be used to trace the source of outbreaks of certain infections.
In recent years nothing has grabbed the attention of the populace world-wide as much as the escalating mortality toll caused by the onslaught of this virus as it continued to spread. Image credit: MIT News
Viruses are the only type of microorganism that cannot reproduce without a host cell. Before entering a cell, viruses exist in a form consisting of two or three distinct parts:- genetic DNA or RNA; a protective coat of protein; a lipid (a type of fatty liquid) envelope around the protein coat when the virus is by itself and not inside a cell. After contacting a host cell, a virus inserts genetic material into the host and continues to reproduce. It is this process that classifies viruses as parasites. The three main shapes of viruses are:
- Helical: An example is the tobacco mosaic virus
- Icosahedral (approximately spherical): Most animal viruses are this shape
- Envelope: Having a protective lipid envelope as mentioned above. An example is HIV.
Other shapes are possible, including non-standard shapes that combine both helical and icosahedral forms.
Viruses exist only to reproduce, spreading to new cells and new hosts in many ways including:
- exchanges of saliva arising from coughing and sneezing
- sexual contact
- contaminated food or water
- insects that carry them from one person to another
Viruses can live on surfaces for some time; usually about three or four days, so if a person touches an object with the virus on their hands, the next person can pick up that virus by touching the same surface.
Some viruses only affect one type of being, say, birds. If a virus that normally affects birds does happen to get into a human, it may pick up some human DNA leading to production of a new type of virus that might be more likely to affect humans in future.
Examples of diseases caused by viruses in humans include:
- the common cold and different types of flu
- measles, mumps, rubella, chicken pox, and shingles
- herpes and cold sores
- Ebola and Hanta fever
- HIV, the virus that causes AIDS
- Severe acute respiratory syndrome (SARS)
- Dengue fever, Zika, and Epstein-Barr
When the immune system of the body detects a virus, it responds to enable cells to survive the attack. The immune system produces special antibodies that can bind to viruses, making them non-infectious. The body also sends T cells to destroy the virus. Most viral infections trigger these types of protective responses from the immune system, but some viruses such as HIV and neurotropic viruses have ways of evading the immune system’s defences. Neurotropic viruses infect nerve cells. They can affect the structure of the central nervous system (CNS) with delayed and progressive effects that can be severe and can cause diseases such as polio, rabies, mumps, and measles. Bacterial infections can be treated with antibiotics, but viral infections require either vaccinations to prevent them in the first place or antiviral drugs to treat them. However, sometimes, the only possible treatment is to provide symptom relief.
Antiviral drugs have been developed largely in response to the AIDS pandemic. These drugs do not destroy the virus, but they inhibit the viral activity inside cells which slows down the progress of the disease. Antivirals are also now available to treat infection with the herpes simplex virus, hepatitis B, hepatitis C, influenza, shingles, and chicken pox. However, wherever possible, the most effective and least costly defence against viruses harmful to humans is by vaccinations. Anti-virus vaccines have succeeded in eliminating diseases, such as smallpox and contain one or more of:
- a weakened form of a particular virus
- viral proteins called antigens which stimulate the body to form antibodies that will fight off future infections from the same virus
- live-attenuated viruses, for example immunisation against poliomyelitis.
There are risks, however, that these attenuated viruses can cause the original disease in people who have weak immune systems.
Vaccinations are now available against polio, measles, mumps, rubella, and several other diseases. Widespread use of these vaccines has reduced their prevalence dramatically, for example two doses of the measles vaccine offers 97% protection against this disease.
Some people, for religious and other reasons choose not to vaccinate their children. In the US they have become known as ‘Antivaxxers’ who ‘help breathe new life into old diseases’.
This article has been an introduction to viruses which are harmful to humans and other living forms. There are, of course, also ‘friendly’ viruses, some of which are helpful such as those which attack or control potentially harmful bacteria such as e-coli in digestive systems. However, most people tend to regard viruses as potentially harmful or even as dangerous entities as evidenced in the wry quotation attributed to the British microbiologist and Nobel laureate, Peter Medawar, describing a virus as ‘a piece of nucleic acid surrounded by bad news’.