The Humble Virus: Friend or Foe?

Life in the Fog

We live surrounded by a fog of viruses. Lurking in that fog is not only SARS-CoV-2, the virus that has tormented the world for the last three years as the cause of the COVID-19 pandemic, but also influenza, measles, and many other pathogenesis. The particles that make up the viral fog are extremely small: a COVID-19-triggering virion measures a mere ten-thousandth of a millimeter in size. A particle this tiny was responsible for bringing about a catastrophe that spanned the globe and subjected humanity to its greatest test since World War II. Lockdowns and isolation caused social chaos and economic stagnation, as governments engaged in a futile game of trial and error while failing to come up with any effective solution. Both in Japan and around the world, the poverty gap widened, and the incidence of starvation in the developing world increased sharply. Even the Olympic games slated to take place in Tokyo in 2020 were postponed.　

While it was the human race’s social sophistication that enabled it to conquer the planet it inhabits, it was that very social organization that enabled COVID-19 to spread, causing us the anguish of being cut off from that organization itself in order to slow the spread of disease. A civilization supposedly protected by modern medicine could do nothing to defend itself against this microscopic virus.

The emergence of new viruses has vexed humanity for some 50,000 years now. To explore this properly, we must first explain the significance of viruses to the human race.

Viruses exist in massive numbers in every corner of the globe. You will find them in deep underground caves, permafrost, hot springs that reach temperatures of over 100°C, deserts, mountains, deep ocean trenches, and even inside the pipes inside nuclear reactors. In fact, it’s more difficult to find a place that does not contain viruses.

Let us now consider why scientists speak of a viral “fog.” Scientists estimate that every cubic meter of atmosphere contains around 8 million viral particles. This means that given the volume of an average-sized room in a Japanese apartment, there will be around 200 million viral particles floating around in that space. A gram of soil contains 1 billion particles, and a cubic centimeter of seawater around 1 million. It is estimated that the total mass of all of the viruses in the world’s oceans is equivalent to that of 75 million blue whales. The scientific journalist Katherine J. Wu, who has a PhD in microbiology from Harvard University, estimates the total number of viral particles on earth to be around 3×10³¹—that’s 3 followed by 31 zeros. In giant number terminology, this amount is expressed as 30 nonillion. By comparison, the number of grains of sand on Earth is estimated at 3²⁰, while the number of visible stars in the sky is estimated at 7×10²². In other words, the number of viral particles on Earth far exceeds the number of grains of sand, or the number of stars in our sky.

A Body Full of Viruses

Viruses live in animals, molds, bacteria . . . in fact, in every type of organism. Viruses themselves were once held to be the only exception to this host rule, until in 2008 scientists analyzed an amoeba taken from a water tank in a Paris office building to discover a virus that lives inside another virus. That virus was named the “Sputnik virus” after the Soviet satellite. There are a total of 9,110 named specimens registered with the International Committee on Taxonomy of Viruses, of which the genetic sequences for around 2,400 have been made public online. Because each type of life form harbors different viruses, a total of well over 10 million varieties of virus may exist on our planet.

The human body is also full of the things. In fact, a team of researchers at the University of California at San Diego led by David Pride estimated there to be over 380 trillion viruses living in the human body—10 times the number of cells in the body. The majority of these viruses are what is known as bacteriophages: viruses that attach to the over 40 trillion bacteria that live in our mouths, intestines, reproductive organs, and skin. These viruses play a role in regulating intestinal flora to prevent overgrowth. However, almost none of the roles of the other viruses in the body are understood.

More evidence for the fact that the human body is a “breeding ground” for viruses is the fact that talking loudly for one minute causes over 1,000 minuscule virus-containing aerosol particles to be spread through the air, where they remain airborne for at least eight minutes. While most of those particles are harmless to humans, should a pathogenic virus find its way into the mix, it could spread to new hosts, conceivably starting another pandemic.

A team of researchers at the University of Tokyo led by Satō Kei was the first to perform an exhaustive study of the viruses that live in a healthy human body. The researchers found that 27 tissue types including the blood, brain, heart, large intestine, lungs, liver, and muscles harbor at least 39 different types of virus. They researchers found eight different viruses living in the brain and nine different viruses living in the heart. The detected specimens included pathogenic viruses like herpes and hepatitis C, as well as friendly viruses that boost the host’s immunity or turn on genes. With this research still in its infancy, there are high expectations for future progress.

Neither Animate nor Inanimate

The debate on whether viruses are alive has yet to reach a conclusion. When a virus was visually observed for the first time in the 1930s following the invention of the electron microscope, most researchers believed viruses to be living organisms. This is because it was already known that viruses cause various illnesses. But when viruses were extracted from organic matter in crystalline form in the mid-1930s, many researchers began to question this. Sometimes behaving like living organisms and sometimes behaving like inanimate objects, viruses defied common sense.

The debate over whether viruses are living or inanimate continues today. While textbooks define lifeforms as requiring cells, metabolism, and the ability to reproduce, the great deal of variety and the many exceptions seen amongst the world’s many known lifeforms has caused researchers in the fields of physics and chemistry to weigh into the debate as well, making the discourse more complicated. For what it’s worth, I believe that viruses are alive. In fact, I once rather mischievously asked a researcher friend who swears black and blue that viruses are inanimate whether the fact that disinfectant is made available to kill the coronavirus means that it is actually “killing” an inanimate object, in response to which he looked perplexed. In other words, the debate is not black and white. I believe that the definition for living organism should be revised, but with an astounding 300 definitions of living organisms proposed, we may first have to define what constitutes a definition.

A Storeroom of Diversity

While their status as harbingers of terrible infectious diseases can make viruses a headache, increasingly, we are finding evidence that viruses are indispensable for biological systems and the ecosystem. For example, when we are infected by a virus, it invades our genes, inserting its own genetic information. In this way, our genome (the entirety of the genetic information contained in our DNA) has been rewritten by viruses, which have thus played a major role in our evolution. Some 8% of the human genome is estimated to be the result of viral insertions.

If this is hard to get your head around, try thinking of a virus as a USB flash drive and a cell as a computer fitted with a USB port of a different specification, preventing the insertion of the drive. After some trial and error, however, the drive is eventually made to fit the port, whereupon the information it contains can be transferred to the computer. Sometimes the contents of the drive overwrite files and infect other computers (the equivalent of a computer virus).

The placenta plays an important role in nurturing the fetus in the belly of a mammal. Researchers have recently discovered that the placenta is composed of virally-introduced genetic information. When, approximately 160 million years ago, a virus-infected gene was introduced to a mammalian precursor, it resulted in the formation of the placenta. This fact was discovered by Ishino Fumitoshi of Tokyo Medical and Dental University, creating a worldwide sensation. The formation of the placenta in the course of mammalian evolution is thus the result of a certain type of infection-acquired virus merging with the cells of the host. The failure of this virus to function in some cases is the reason that the marsupials that are endemic to Australia and North America do not have a placenta, rather nurturing their young in an external sac.

In the year 2000, the revelation was made that the fetus is protected by viruses, once more revolutionizing attitudes to them. The fetus inherits genetic traits from both parents. However, genetic material from the father, which makes up half of the fetus’s genome, are recognized by the mother’s immune system as foreign bodies, meaning that the mother’s body would be expected to reject the fetus in the same way it might reject a transplanted organ, preventing the survival of the fetus. And yet, fetuses are able to grow and be born. For a long time, this remained a major riddle. The answer lies in viral activity. Viruses that live in the human body activate to create a cellular membrane made of proteins (called a “syncytiotrophoblast”) that envelops the fetus. This membrane lets through the nutrients and oxygen that are necessary for fetal development while protecting the fetus by keeping out the mother’s lymphocytes, which mediate cellular rejection by the mother’s immune system.

It has been established that viruses play an important role in the ecosystem as well. When worker bees are infected by a specific virus, they become aggressive, and boldly fight their natural enemy, the Asian giant hornet. Viruses also regulate the “red tide” that can form in the ocean as a result of the overgrowth of plankton. There is also evidence that viruses are significantly involved in the carbon cycle that causes global warming. Curtis Suttle, a virologist from the University of British Columbia, has declared that without viruses there would be no humans or other forms of life.

As the significance of the existence of viruses becomes increasingly apparent, I am preoccupied by the idea that we have opened very slightly the door to an enormous viral world that exists within our ecosystem, giving a glimpse into a previously undiscovered and unvisited “storeroom” of genetic diversity. This storeroom could contain information on antigens, antibodies, and proteins, and even the secret to evolution. It might even become a valuable asset for humanity that leads us to the cures for cancer and rare diseases.

My hope is that we will see greater clarity in the ongoing battle between infections and human civilization.

(Originally published in Japanese. Banner photo: A field hospital set up in a sports facility in the Santo André district of São Paulo, Brazil, in May 2021 as that country’s death toll exceeded 435,000. © Mario Tama/Getty Images.)