What Exactly Is a Virus?

Viruses are themselves an enigma that exist on the edges of life. That’s how John Barry describes viruses in his excellent The Great Influenza, which describes the events around the 1918 Spanish Flu. Barry has the knack for explaining biological concepts. In the era of the coronavirus, I thought it would be good to know what a virus actually is and his description is one of the best I’ve read. I’ve extracted the relevant parts from the book and you’ll see what I mean:

Not bacteria

“Viruses are themselves an enigma that exist on the edges of life. They are not simply small bacteria. Bacteria consist of only one cell, but they are fully alive. Each has a metabolism, requires food, produces waste, and reproduces by division. Viruses do not eat or burn oxygen for energy. They do not engage in any process that could be considered metabolic. They do not produce waste. They make no side products, by accident or design. They do not even reproduce independently. They are less than a fully living organism but more than an inert collection of chemicals.”

One mission

“A virus has only one function: to replicate itself. But unlike other life forms (if a virus is considered a life form), a virus does not even do that itself. It invades cells that have energy and then, like some alien puppet master, it subverts them, takes them over, forces them to make thousands, and in some cases hundreds of thousands, of new viruses. The power to do this lies in their genes.”


“In most life forms, genes are stretched out along the length of a filament-like molecule of DNA. But many viruses—including influenza, HIV, and the coronavirus—encode their genes in RNA, an even simpler but less stable molecule.”

“Genes resemble software; just as a sequence of bits in a computer code tells the computer what to do—whether to run a word processing program, a computer game, or an Internet search, genes tell the cell what to do. DNA and RNA are strings of these chemicals.”

The takeover

“When a gene in a cell is activated, it orders the cell to make particular proteins. Proteins can be used like bricks as building blocks of tissue. When a virus successfully invades a cell, it inserts its own genes into the cell’s genome, and the viral genes seize control from the cell’s own genes. The cell’s internal machinery then begins producing what the viral genes demand instead of what the cell needs for itself.”

“So the cell turns out hundreds of thousands of viral proteins, which bind together with copies of the viral genome to form new viruses. Then the new viruses escape. In this process the host cell almost always dies, usually when the new viral particles burst through the cell surface to invade other cells. The results of this binding can be as dramatic, or destructive, as any act of sex or love or hate or violence.”


“Whenever an organism reproduces, its genes try to make exact copies of themselves. But sometimes mistakes—mutations—occur in this process. This is true whether the genes belong to people, plants, or viruses. The more advanced the organism, however, the more mechanisms exist to prevent mutations.”

“A person mutates at a much slower rate than bacteria, bacteria mutates at a much slower rate than a virus—and a DNA virus mutates at a much slower rate than an RNA virus. DNA has a kind of built-in proofreading mechanism to cut down on copying mistakes. RNA has no proofreading mechanism whatsoever, no way to protect against mutation.”


“So viruses that use RNA to carry their genetic information mutate much faster—from 10,000 to 1 million times faster—than any DNA virus. Different RNA viruses mutate at different rates as well. Even the viruses produced from a single cell will include many different versions of themselves. Most of these mutations interfere with the functioning of the virus and will either destroy the virus outright or destroy its ability to infect. But other mutations, sometimes in a single base, a single letter, in its genetic code will allow the virus to adapt rapidly to a new situation. It is this adaptability that explains why these mutant swarms, can move rapidly back and forth between different environments and also develop extraordinarily rapid drug resistance.”

Scary stuff!


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