Viruses are infectious, tiny and nasty. But are they alive?
Not really, although it depends on what your definition of "alive" is, two infectious disease doctors told Live Science.
Living beings, such as plants and animals, contain cellular machinery that allows them to self-replicate. In contrast, viruses are free forms of DNA or RNA that can't replicate on their own.
Rather, viruses need to invade a living organism to replicate, said Dr. Otto Yang, a professor of medicine and microbiology, immunology and molecular genetics at the David Geffen School of Medicine at the University of California, Los Angeles.
"[Viruses are] packaged RNA or DNA," Yang told Live Science. "They make more copies of themselves by hijacking the machinery of cells to replicate themselves."
Is it alive?
Countless philosophers and scientists have debated how to define whether something is alive. According to the seven characteristics of life, all living beings must be able to respond to stimuli; grow over time; produce offspring; maintain a stable body temperature; metabolize energy; consist of one or more cells; and adapt to their environment.
However, some life-forms don't fit every single characteristic. Most hybrid animals, such as mules (a cross between a donkey and a horse), can't reproduce because they are sterile. Moreover, rocks can grow, albeit in a passive way, with new material flowing over them. But this classification problem goes away when a simpler definition of "life" is used.
"Take a cat, a plant and a rock, and leave them in a room for days," said Amesh Adalja, an infectious disease physician and an affiliated scholar at the Johns Hopkins Center for Health Security in Baltimore. "Come back, and the cat and the plant will have changed, but the rock will essentially be the same," he said.
Like a rock, most viruses would be fine if they were left indefinitely in a room, Adalja said. In addition, he noted that living beings have self-generated and self-sustaining actions — meaning they can seek out sustenance and behave in self-preserving ways. In other words, "they're taking actions to further their lives, [such as] a plant sprouting its roots to find water or an animal looking for food," Adalja said.
Something that is not alive, such as a virus, does not have self-generated or self-sustaining actions, he said.
"I don't think viruses qualify as being alive. They are, in essence, inert unless they come into contact with a living cell," Adalja said. "There are some characteristics of viruses that put them on the borderline [of being alive] — they have genetic material: DNA or RNA. It's not the same thing as a rock, but it's clearly not the same thing as even bacteria, in terms of that self-sustaining and self-generated action."
Yang agreed, saying, "Without a cell, a virus cannot reproduce. And so from that standpoint, it's really not alive, if you consider life to be something that can reproduce by itself independently."
However, "if you loosen up your definition of life to something that can make copies of itself with help, then you could call it alive," Yang said.
It's thought that some of the very first life-forms on Earth were RNA molecules, as "RNA molecules, under the right conditions, can make copies of themselves," Yang said. "Viruses maybe evolved from that ancestor, but lost the capability to self-replicate."
Originally published on Live Science.