They are as old as life itself, but scientists cannot say for sure if they are alive. Viruses are embedded in our DNA, shaping the human saga through mutation and resistance. We touch hundreds of millions of them every day. As the new coronavirus epidemic disrupts global markets and prompts health authorities to take unprecedented containment measures, a very fundamental question needs to be asked: what exactly is a virus? What are they made of? Where do they come from? And, perhaps more importantly, why are they trying to kill us?
The story of viruses is perhaps best told through hallucinating numbers. According to Curtis Suttle, a virologist at the University of British Columbia, Canada, the physical properties of viruses make them difficult to understand.
Their small size, for starters. If every virus in a human body were the size of a pinhead, the average adult would be 150 kilometres tall. In a 2018 study, Curtis Suttle found that more than 800 million viruses were deposited on every square metre of Earth every day. In a tablespoon of seawater, there are usually more viruses than people in Europe. « We swallow over a billion viruses every time we go swimming« Suttle says. « We're awash in viruses"
A 2011 paper published in Nature Microbiology estimated that there are more than one quintillion - a follow-up of 30 zeroes - of viruses on Earth. Put them all together and they would stretch over 100 million light years, or 1,000 times the width of the Milky Way.
The virus as a concept
According to Teri Shors, professor of biology at the University of Wisconsin Oshkosh and author of several books on the subject, viruses are best thought of as "molecular packages. « These packagings must be small enough to fit inside a cell to cause infection.« she told AFP.
Essentially chains of genetic material contained by a few protein molecules, viruses occupy a strange middle ground between the living and the inert. Because they have no cells and do not produce energy through respiration - a key definition of living organisms - many scientists do not consider them alive. Yet as soon as they enter their host, viruses become active in a way rarely seen in nature, hacking into cells with new genetic instructions to replicate themselves at breakneck speed.
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Ed Rybicki, a virologist at the University of Cape Town, calls viruses "as much a concept as a thing. « I consider viruses to be alive, because when they're in a cell, they ARE the cell...« he told AFP.
Teri Shors describes viruses as "metabolically inactive". « Unless they can penetrate a warm body and enter a cell, viruses are inert...« she adds. But once it infects its host, " the entire cellular machinery is entirely devoted to the production of viral offspring« notes Curtis Suttle.
To the origins of life
Although their beginnings are uncertain, viruses have left their mark on almost all life on Earth, including humans. About eight percent of the human genome is viral in origin - the remnants of ancient viruses that infected us, developing species-wide tolerance.
But their story begins long before that of the humans. « We think the viruses were there at the very beginning...« comments Curtis Suttle. « Whatever primordial soup gave rise to cell life, it probably gave rise to viral life at the same time."
Are all viruses bad?
Most viruses attract our attention because they make us sick. In recent years, outbreaks of viral infectious diseases have become widespread, from the current coronavirus epidemic to the SARS epidemic in the early 2000s in Ebola in West and Central Africa.
But there are also virtuous viruses. « Almost all viruses are actually harmless to humans.« relieves Rybicki. Many viruses are beneficial to human health, infecting other organisms that would otherwise harm us.
Another advantage: the absorption of carbon from oceanic algae, which helps to purify the air we breathe, is strongly accelerated by viruses.
And they also have health applications. In addition to vaccines derived from weakened or killed viruses, a new area of treatment, virotherapy, is developing new ways to treat chronic diseases such as cancer. « These viruses replicate in cancer cells but not in healthy cells, so this treatment is not as toxic as conventional cancer therapies.« says Teri Shors.
For Rybicki, who has spent most of her professional life trying to unravel their secrets, the most remarkable thing about viruses is how many mysteries they still hold. « They are the most diverse organisms on our planet (...) and we still know nothing about them..."
The recent discovery of giant viruses, the megavirus...shows that they're still capable of surprising us.
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