While the ongoing COVID-19 pandemic may make it difficult to perceive viruses as anything other than agents of destruction, the mundane truth is that viruses are barely even living things. In fact, some biologists — most, even — do not believe that they actually count as living things.
How can this be so? After all, viruses like SARS-CoV-2, the virus that causes COVID-19, seem to have a horrifically efficient mechanism for reproducing themselves, suggesting they have a will to live. How could something with such a will to live not be — well — alive?
As it turns out, the question of whether a virus should be considered "alive" is one of the most fundamental in modern biology. Even today, scientists do not have a concrete answer — though they all have opinions.
"There is no single, consensus definition of life, and I doubt it is possible to develop a fully satisfactory one," Dr. Eugene V. Koonin, the Evolutionary Genomics Group Leader and Distinguished Investigator at the National Institutes of Health, told Salon by email. Koonin said that while many scientists consider this question to be a "pointless, pseudo-philosophical exercise," Koonin does not personally perceive the question that way. Rather, it is "both interesting and useful for understanding the foundations of biology, and in a purely operational sense, to recognize life forms if and when candidates are discovered outside Earth," Koonin said.
"By some accounts, viruses make up the most biomass on the planet. So, I think they are alive."
Scientists do agree on these points: viruses are biological entities with a protein coat on their outside and nucleic acid molecules (like DNA or RNA) on their inside. In order to function and reproduce, viruses need to infect living cells and co-opt their genetic material. Viruses have played a huge role in the evolution of life, even of humans: sometimes, after infecting someone, they leave behind pieces of their genetic material in our DNA. It is estimated that 8 percent of the human genome consists of genetic material from viruses that infected us at one point; their remnants in our DNA are believed to have a protective effect.
In considering whether these tiny agglomerations of proteins and either RNA or DNA are technically alive, Koonin cited NASA's definition for life: "Life is a self-sustaining chemical system capable of Darwinian evolution."
Koonin argued for a somewhat more expensive definition, suggesting: "A life form is a self-reproducing system of chemical reactions endowed with a dedicated, replicating memory storage device that directs the formation of the system's components."
The essential characteristic of this definition, Koonin put forward to Salon, is that it holds all life forms to include memory storage (genome, replicator) and a metabolic network (cell, reproducer). If his definition is correct, then that would mean viruses don't count as living.
"Viruses are pure replicators that fully depend on the host (cells) for metabolism and so, under this definition, do not qualify as life forms," Koonin told Salon.
Dr. Jason Shepherd, an associate professor of neurobiology at the University of Utah Medical School, does not share Koonin's assessment about whether viruses qualify as alive.
"This is a classic question in biology, with the classic definition that something is alive if it reproduces, grows, and responds to external stimuli," Shepherd wrote to Salon. "The reason why people don't think viruses are alive is that they are parasites that need host cells to replicate."
Yet the genetic material at the center of each virus's protein shell is either DNA or RNA, the same molecules that create human beings.
"Some are very sophisticated machines that are extremely successful at propagating themselves," Shepherd added. "Indeed, by some accounts, viruses make up the most biomass on the planet. So, I think they are alive."
"The fact that they can live in cells and replicate in cells, in my opinion, that makes them alive."
Dr. Tracey Goldstein also thinks viruses are alive. Goldstein — who works for USAID, is currently an adjunct professor at the University of Illinois and is formerly a professor at the Department of Pathology, Immunology and Microbiology and Associate Director of the One Health Institute at the University of California, Davis — describes viruses to Salon as "internal parasites," adding that although she thinks they should be considered alive, there is the qualifier that "they don't have the ability to live independently." Yet Goldstein does not consider that caveat to be insurmountable in terms of the question of viral life.
"Most viruses can survive outside of cells for short amounts of time, and some viruses such as herpes or influenza can survive on surfaces, but the fact that they can live in cells and replicate in cells, in my opinion, that makes them alive," Goldstein told Salon. It is helpful, because viruses are fundamentally parasitical, to understand their day-to-day existence through a framework similar to that used for comprehending other parasites.
"We see parasites in many different ways," Goldstein observed, citing as examples fleas that need to stay on dogs to survive, "but it's still alive." Similarly, in the plant world, we see species like orchids or moss "that live on other plants in order to get nutrients, but they're still considered alive." In the end, "there are different definitions of level of functionality or how they might relate to the world," but these plants are still ultimately deemed to be alive.
By contrast, Dr. Stanley Perlman, a professor of microbiology and immunology, as well as of pediatrics, at the University of Iowa, is decisively against the idea that viruses are alive.
"Viruses cannot grow outside of an infected cell so I do not think that they are alive," Perlman told Salon by email. "The problem comes with some pathogens that require mammalian cells to replicate, but are considered alive."
Reviewing the evidence and prevailing theories, it seems that Perlman's view may become less popular. As scientists learn more and more about viruses, the case for classifying them as living becomes increasingly strong. Shepherd, for his part, expressed excitement over the discovery of so-called "giant viruses" that can be bigger than bacteria and argued that the question could be put to rest "if your definition of life is that they should be able to replicate on their own" since that is "testable." At the same time, "no known virus has been shown to do this," meaning that for now the question remains at best unresolved.
Koonin, for his part, does not seem entirely sure that that is a bad thing.
"I do not believe that a demonstrably accurate definition of life and accordingly a fully objective, defendable answer to the question 'Are viruses alive?' are attainable," Koonin wrote to Salon. "There can be no experiment or rigorous theory to address these questions. We only seek most useful and consistent solutions."