Paris (AFP)

They are as old as life itself, but scientists cannot say for sure if they are alive. Viruses are written into our DNA, shaping the human saga through mutation and resistance.

We touch hundreds of millions every day.

As the new coronavirus epidemic is disrupting global markets and prompting health authorities to take unprecedented containment measures, a very basic 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?

- Unimaginable figures -

Curtis Suttle, a virologist at the University of British Columbia in Canada, says the physical properties of viruses make them difficult to understand.

Their small size, to start with. If each virus in a human body reaches the size of a pinhead, the average adult would reach 150 kilometers (95 miles) in height.

In a 2018 study, Suttle discovered that more than 800 million viruses deposit on every square meter of Earth every day.

In a tablespoon of seawater, there are generally more viruses than people in Europe.

"We swallow over a billion viruses every time we go swimming," says Suttle. "We are inundated with viruses".

A 2011 article published in Nature Microbiology estimated that there was more than a quintillion - a follow-up of 30 zeros - of virus on Earth.

Put them all end to end and they would stretch for 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 packages 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.

Since they have no cells and do not produce energy through respiration - a key definition of living organisms - many scientists do not consider them to be living.

However, as soon as they enter their host, viruses enter into activity in a way rarely seen in nature, hacking cells with new genetic instructions to replicate at breakneck speed.

For Ed Rybicki, virologist at the University of Cape Town, viruses are "as much a concept as a thing".

"I consider that the viruses are alive, because when they are in a cell, they ARE the cell", he explained to AFP.

Teri Shors describes viruses as "metabolically inactive".

"Unless they can get into a hot body and get inside a cell, viruses are inert," she adds.

But once it infects its host, "the whole cellular machinery is entirely devoted to the production of viral descendants," remarks Curtis Suttle.

"It's the living virus".

- Origins -

Although their beginnings are uncertain, viruses have left their mark on most of life on Earth, including humans.

About eight percent of the human genome is of viral origin - that is, the remains of old viruses that infected us, developing tolerance at the species level.

But their story begins long before that of humans.

"We think the viruses were there at the very beginning," said Curtis Suttle.

"Whatever primordial soup that gave birth to cell life, it probably gave birth to viral life at the same time."

- Are all viruses bad? -

Most viruses catch our attention because they make us sick.

In recent years, outbreaks of viral infectious diseases have become widespread, from the current coronavirus epidemic to that of SARS in the early 2000s at Ebola in West and Central Africa.

But there are also virtuous viruses.

"Almost all viruses are actually harmless to humans," said Rybicki.

Indeed, 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 purify the air we breathe, is greatly accelerated by viruses.

And they also have health apps.

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," said Teri Shors.

For Rybicki, who has spent most of his professional life trying to unlock their secrets, the most remarkable thing about viruses is the number of mysteries they still hold.

"They are the most diverse organisms on our planet (...) and we don't know anything about them yet."

The recent discovery of giant viruses shows that they are still capable of surprising us.

© 2020 AFP