This is what the Spanish flu virus looked like, which killed 50 to 100 million people in 1918. If all viruses suddenly disappeared, the world would be very different – and not necessarily better.
What would become of us without viruses? And what does “kill the winner” mean? Looking at the horrifying images of the Covid-19 pandemic unfolding before the eyes of the world thanks to the media and social networks, you might think that viruses exist only to bring humanity to its knees and kill as many people as possible.
Over the past millennium, diseases and their resulting epidemics have claimed countless lives. Some of these viruses have killed a significant portion of the world’s population: the Spanish flu epidemic of 1918 killed an estimated 50 to 100 million people. In addition, smallpox alone is believed to have killed an additional 200 million people in the 20th century. And the current Covid-19 pandemic is just another in an endless series of deadly viruses that have struck humanity.
Most of us today, if given a magic wand and offered to wave it to get rid of all the viruses on the planet, would gladly agree. I’m afraid that would be a fatal mistake. More deadly, in fact, than any of the nastiest viruses. “If all viruses suddenly disappeared, the world would be beautiful – for about a day and a half. And then we’d all die, that’s all,” says Tony Goldberg, an epidemiologist at the University of Wisconsin-Madison. “The important things that viruses are responsible for far outweigh the damage they do.”
In general, says Susan Lopez Chariton, a virologist at the National Autonomous University of Mexico, “without viruses, our end is near.
Some viruses keep fungi and plants healthy. Most people don’t even realize the role viruses play in life on Earth, focusing only on those that kill us. Almost all virologists study only pathogens, and only recently have a few scientists decided to study the viruses that keep us and our planet alive. Thanks to this small group of researchers, we may get a more balanced view of the world of viruses. It turns out that there are good ones out there, and most of them are actually good.
But one thing scientists already know for sure is that without viruses, our planet as we know it would cease to exist. And even if we set ourselves the goal of eradicating all viruses from the planet, it is virtually impossible. But by imagining what the world would be like without viruses, we can better understand how important they are to our survival, and how much we still have to learn about these microscopic, simplest forms of life.
Without viruses, our planet would cease to exist. First of all, it should be said that scientists do not even know how many viruses there are. Thousands are officially classified, but there are millions. “We’ve only opened up a small part of it because we’re not particularly interested in it,” says Marilyn Rusink, a viral ecologist at Penn State University. “That’s a biased attitude: science has always been primarily interested in pathogens.” Scientists do not know exactly what percentage of all viruses are dangerous to humans. “If you look at large numbers, the percentage of dangerous viruses is statistically close to zero,” says Curtis Suttle, a virusologist and ecologist at the University of British Columbia. “Almost all of the viruses that exist are not pathogenic to us.”
At least we know that phages (bacteriophages, viruses that selectively attack bacterial cells) are incredibly important. Their name comes from the Greek word for “to devour,” and that is exactly what they do. “In the bacterial world, they are the top predators,” Goldberg says. “Without them, we would be in trouble.” Phages are the main regulators of bacterial populations in the ocean, and probably in all other ecosystems on our planet. If phages suddenly disappeared, some populations would likely explode and suppress others, which would stop growing altogether. For the ocean, this would be a particularly serious problem because more than 90% of all living organisms (by total mass) in the ocean are microorganisms. And these microbes produce about half of all the oxygen on the planet – a process made possible by viruses.
In the ocean, 90% of all living organisms are microorganisms. We explain quickly, simply, and clearly what happened, why it matters, and what will happen next. The number of offers should remain: Episodes End of story: Podcast Advertising. These viruses destroy about 20% of all oceanic microorganisms and about 50% of all oceanic bacteria every day. They provide enough nutrients for oxygen-producing plankton to support life on the planet. “If there is no death, there is no life, because life depends entirely on the recycling of materials,” Sattl points out. “Viruses are very important for this recycling. Researchers studying insect pests have also found that viruses are critical to population control. When some species start to overgrow, “a virus comes and destroys them,” Russink says. It is a very natural process for ecosystems. This process is called “killing the winner” and is quite common in many other species, including our own – the pandemic is proof of this. “When the population gets too big, viruses reproduce incredibly fast and shrink their size, making room for everything else to live,” Sattl points out. If all viruses suddenly disappear, the most competitive species will thrive at the expense of all others. “We will quickly lose a significant portion of our planet’s biodiversity,” says Sattl. “Only a few species will thrive, while the rest will become extinct.”
According to experts, without viruses, our planet would lose a significant portion of its biological diversity. For some organisms, viruses are critical for survival or to gain a competitive advantage. For example, scientists suspect that viruses play an important role in helping cows and other ruminants convert cellulose from grass into sugars, which are metabolized and eventually turned into milk, and in helping them gain body mass. Researchers believe that viruses are important for maintaining a healthy microbiome in the human and animal body. “These things are not yet fully understood, but we are finding more and more examples of such close interaction with viruses as an essential element of the ecosystem,” says Sattl. Russink and her colleagues have found solid evidence of this. In one of their studies, they worked with a colony of microscopic fungi living with a particular type of grass in Yellowstone National Park (a biosphere reserve in the United States, famous for its geothermal landscape and geysers – BBC note) and discovered that a virus infecting the fungi enabled the grass to better withstand geothermal soil temperatures. “If all three elements are present – virus, fungus and grass – then grass can grow on hot soil,” says Russink. “A fungus without a virus is not able to do that.”
In Yellowstone National Park, some grasses have become more resistant to high temperatures – thanks to a virus. Russink and her colleagues have discovered that fungi usually transmit viruses “by inheritance” – from generation to generation. And although scientists have not yet been able to determine the function of most of these viruses, it is safe to assume that they are somehow helping the mushrooms. “Why else would plants cling to them?” – Russian wonders. And if all these beneficial viruses disappear, the grasses and other organisms in which they currently live will weaken and possibly die.
Infecting the human body with certain harmless viruses actually helps to fight off certain pathogens. GB type C virus, a common non-pathogenic human virus (unlike its distant relatives, West Nile virus and dengue fever virus), has been associated with slowing the progression of HIV in HIV-infected individuals. In addition, scientists have discovered that people with GB virus type C are less likely to die if they become infected with the Ebola virus. Similarly, herpes makes mice less susceptible to certain bacterial infections, including bubonic plague and listeriosis (a common type of food poisoning). Of course, it is unethical to perform similar experiments on humans infected with herpes, bubonic plague, and listeriosis, but the authors of the study suggest that humans would have a similar picture.
The herpes virus makes mice – and potentially humans – less susceptible to certain bacterial infections. It seems that without viruses, both humans and many other species would be more susceptible to various diseases. Viruses are also one of the most promising therapeutic agents for certain diseases. Phage therapy (the treatment of infectious patients and carriers with bacteriophage preparations), which began in the Soviet Union in the 1920s, uses viruses to destroy bacterial infections. Today, it is a rapidly developing area of scientific research. Not only because of the increasing resistance of pathogens to antibiotics, but also because bacteriophages can be precisely targeted to specific types of bacteria – unlike antibiotics, which kill all bacteria indiscriminately. “When antibiotics fail, viruses save people’s lives,” Sattl says. Oncolytic viral therapy for cancer, in which only cancer cells are infected and destroyed, is also less toxic and more effective than other methods of cancer treatment. Targeting destructive bacteria or cancer cells, therapeutic viruses act as “microscopic guided missiles that hit the target with precision,” Goldberg says. “We need viruses that will take us to a new level of therapy, a new generation of therapy.”
Because viruses constantly mutate and replicate, they represent a vast reservoir of genetic innovation that can be exploited by other organisms. Viruses invade the cells of other organisms and take over their means of reproduction. If this happens in the germ line cell (egg and sperm), the viral code can be passed from generation to generation and become a permanent part of it. “All organisms that can be infected by viruses have the ability to accept viral genes and use them to their advantage,” notes Goldberg. “Incorporation of new DNA into the genome is the primary mode of evolution.” In other words, the disappearance of all viruses will affect the evolutionary potential of all life on our planet. Including Homo sapiens. Viral elements make up about 8% of the human genome, while mammalian genomes in general are spiced with about 100,000 gene remnants that once belonged to viruses. Virus code is often an inactive part of DNA, but sometimes it gives the organism new, useful and even important properties. In 2018, for example, two research teams independently made an astonishing discovery. A gene of viral origin encodes a protein that plays a key role in the formation of long-term memory by transmitting information between nerve cells.
It is the ancient retroviruses that are responsible for the fact that humans are capable of viviparity. The most remarkable example, however, concerns the evolution of the mammalian placenta and the temporal expression of genes during human pregnancy. There is evidence that our ability to give birth to live children is due to a portion of the genetic code taken from ancient retroviruses that infected our distant ancestors over 130 million years ago. Here’s what the authors of this discovery wrote in the journal PLOS Biology in 2018: “It is very tempting to speculate that human pregnancy might have occurred very differently (or not at all) if many epochs of retroviral pandemics had not affected our ancestors during evolution.”
Experts believe that such genetic code particles can be found in all forms of multicellular life. “They probably have many functions that we know nothing about,” says Sattl. Scientists have only begun to discover the ways in which viruses help sustain life. Ultimately, the more we learn about all viruses (not just those that cause disease), the better equipped we will be to use certain viruses for peaceful purposes and to develop effective defenses against other viruses that could lead to another pandemic. In addition, studying the rich viral diversity will help us gain a deeper understanding of how our planet functions, its ecosystems and organisms. According to Sattl, “We need to make an effort to understand what is happening and what is in store for us – for our own benefit.”
More articles on related topics – on the BBC Future website.