How viruses blur the boundaries of life

When people talk about the coronavirus, they sometimes describe this invisible entity as if it has a personality and even a conscience. If you ask a biology or medical student what a virus is, they will tell you that a virus is not a living organism, or at most that it exists at the border between living and dead – a kind of walking dead.

For biologists who specialise in virology, however, this view is not clear-cut. Scientists still disagree on whether viruses are truly alive or not.

What scientists can agree on is that a virus adapts to new conditions, evolves and sometimes harms humans. It is also an infectious agent that can only replicate within a host organism such as bacteria, plants or animals.

The boundary between being alive and dead is a concept with no specific criteria. So to help you think about whether viruses are alive, I will talk you through some of the different definitions of life in science.

Throughout history, scientists have debated the definition of life and researchers from different fields still disagree. This debate shapes scientific understanding and influences public health decisions – for example, defining whether viruses are “alive” affects how we design vaccines and strategies to stop their spread.

Biologists may refer you to Erwin Schrödinger’s definition of life. Schrödinger was an Austrian Nobel-prize winning physicist who published a book in 1944 called What is Life? He was one of the first scientists to try to define life and is perhaps better known in popular culture for his “Schrödinger’s cat” thought experiment.

He proposed that life is a form of negative “entropy”, a scientific concept that explains how disordered something is. A physical system will always increase in entropy/disorder unless we insert energy to change this process. Schrödinger thought living things create and maintain order by using energy.

For example, a messy bedroom doesn’t clean itself, but a person can tidy it. Organisms do something similar at the molecular level. DNA is highly structured, allowing it to store genetic information. Proteins fold into specific shapes to function properly. In contrast, after an organism dies, its molecules break down, increasing disorder.

Schrödinger later revised his view – around the 1950s – suggesting that life depends on free energy. Free energy is the energy that drives chemical reactions in living things. This marked a shift from focusing on order (negative entropy) to emphasising energy as essential for life.

In the mid-20th century, scientists switched from defining life to describing its key characteristics. Studying organisms such as bacteria, plants and animals, they identified common traits, setting a precedent still followed today.

Rather than seeking a single definition, researchers classify entities based on these traits. To decide whether a virus is alive, researchers assess how well it meets these criteria.

According to biology, the smallest unit of life is the cell. A cell is an independent unit which makes functional molecules (such as proteins and enzymes). Cells can use their own molecules to replicate genetic material independently. A virus also has genetic material but needs to use the host cell’s enzymes to make functional molecules or replicate its genetic material.

Put simply, a virus does not replicate or function independently. So by the biological definition, a virus cannot be categorised as a living organism.

But from a genetic and evolutionary point of view a living organism is defined by its ability to reproduce. A person who does not have children is still considered to be alive as they are part of the gene pool and descended from people who did have children. From this view a virus is alive, since it can produce similar offspring.

Some scientists also focus on metabolism and energy production as criteria for life. Metabolism includes catabolism (breaking down molecules like sugars during digestion) and anabolism (building molecules like muscle tissue), linking energy and material. These reactions require molecular structures to generate or use energy – structures viruses lack.

Does that mean viruses aren’t alive? An amoeba, for instance, uses nutrients and enzymes to sustain itself, while viruses rely entirely on a host. From this perspective, viruses don’t meet the metabolic criteria for life. However, some argue that since viruses hijack a host’s metabolism to replicate, they show life-like behaviour.

If we consider nutrients to be sources of free energy, a cell uses energy from the environment to build what it needs. As the cell absorbs energy from the environment, it builds and maintains its internal structures – like proteins and membranes.

It also releases a byproduct – carbon dioxide – that contributes to disorder in the external environment. Viruses also do this. They make their structures by using the external environment, a host cell in this case. The viruses’ byproducts may be what makes us sick.

As we explore the complexities of biology, it becomes clear that defining life itself is anything but straightforward. Viruses display both life-like and non-living traits, which influences how we approach treatments like antiviral drugs designed to block their replication inside host cells.

Heshmat Borhani does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.