Plants and animals appear to make up most of the living material on Earth. This is not actually the case.
We are biased in our view of the living world by size. Larger objects capture our attention while smaller organisms pass unnoticed. As a consequence, the 75 per cent of the biomass which is bacteria and other single-celled creatures is largely invisible, living under the radar.
But we are ultimately dependent upon micro-organisms for many aspects of life as we know it. For example, our gut microbiome allows us to digest our food while bacteria in the soil provide nitrogen for all living creatures. Even the oxygen we breathe is largely a result of composite micro-organisms.
There is one class of micro-organisms, though, which can be generally classified as good for nothing parasites – viruses. Simply put, viruses can only reproduce through the machinery of a host organism. They are tiny capsules containing little more than DNA or RNA and they survive only by hijacking another organism’s metabolism.
Recent outbreaks of disease, such as the 2014 Ebola outbreak in West Africa, demonstrate just how effective viruses are at achieving their “goal” – reproduction – and how devastating they are to the host organism resulting in thousands of deaths.
Viruses are not a recent evolutionary phenomenon. They have been around for billions of years infecting everything from single celled proto-bacteria to giant creatures such as tyrannosaurus rex and the blue whale. No species is immune to viral infection.
For much of human history, they went entirely un-detected. It was only in the 1800s that we began to gain an understanding of the microscopic world. Pasteur’s germ theory laid the basis for a new approach to medicine although not all diseases.
The discovery of viruses could be credited to a German chemist and agricultural researcher Adolf Mayer. He was interested in the cause of tobacco mosaic disease. He crushed the leaves of diseased plants and was able to infect healthy plants with the extract demonstrating the diseases was transmittable. However, he was unable to find any bacteria in the extract nor was he able to cause the disease with any known bacteria.
A few years later, in 1892, Dmitri Ivanosky, a Russian student, undertook a similar experiment but used a filter specifically designed to sieve out bacteria. The resulting extract still caused tobacco mosaic disease so he hypothesized the presence of a toxin in the solution.
It wasn’t until 1898 that Martinas Beijerinck recognized the living liquid virus was too small to be caught by filtration. And it wasn’t until 1939 that the tobacco mosaic virus was first imaged using the newly invented electron microscope.
Viruses are very small. An influenza virus is 130 nm while a rhinovirus is only 30 nm. Compare this with a red blood cell at 8,000 nm in size or a human skin cell at 30,000 nm and it is easy to understand why they went unnoticed for so long.
It is a virus’ simplicity which allows it to be so small. It is essentially little more than a stretch of either DNA or RNA, single-stranded or double-stranded, wrapped up in a protein shell called a capsid. It lacks the machinery we normally associate with life – the ability to transcribe DNA into RNA and to translate RNA into protein. It is little more than a blueprint.
Virus reproduce by infecting a host cell, injecting their genetic coding, and taking over the molecular machinery (sometimes to the exclusion of the cell’s natural function). Its sole purpose is to make many, many copies of itself which are then released either into the host or the environment through the host.
It is usually during the release phase where we notice the effects of a virus as it tends to destroy cells, resulting in discharges into the lungs or nasal passages or through the blood vessels. An effective virus is neither instantly nor completely lethal. It needs host organisms and time to reproduce, so it does not kill everyone who is infected right away.
Which brings us to the coronavirus which has world health experts worried. COVID-19 appears, so far, to be both high infectious, which means a large number of people will likely develop the disease, and only moderately lethal, resulting in death in about one per cent of the cases.
If its infection rate is maintained, it is estimated 60 per cent of the world’s population could catch the disease. With a one per cent mortality rate, this would result in around 48 million deaths, far more than previous pandemics such as the 1918-1919 Spanish Flu.
When a virus can be spread by touch or contact with a contaminated surface hours after an infected individual has been present, a global disaster is a real and present danger. All by an organism far too small to see.