What are viruses, how do they work?

Dear Doctor: Everybody is talking about the coronavirus right now, but I still don’t really know what a virus is. How do they work? Why don’t antibiotics kill them?

Dear Reader: You’re far from alone in your struggle to understand viruses. They are simultaneously simple and quite complex, and so small that it wasn’t until the invention of the electron microscope almost 90 years ago that we were even able to see them.

A virus is neither plant nor animal; it isn’t a bacterium, fungus or one-celled organism; and it can’t live or reproduce outside of a living host cell. Considering all that, it’s not surprising that the scientific community continues to debate whether or not viruses are even alive. In fact, it’s almost easier to talk about what a virus isn’t than to explain what it is. But we’ll do our best.

A single virus particle is known as a virion. It’s a packet of genetic material — either DNA or RNA — wrapped in a layer of proteins, known as a capsid. In many kinds of viruses, the protein shell is topped by a layer of lipids, a type of organic compound that’s roughly comparable to fat and is not soluble in water. Taken together, the protein and lipid structure is known as an envelope.

Viruses are mind-bendingly small. They range from about 20 nanometers to upward of 300 nanometers in size, with many tending toward the lower end of the scale. As a point of reference, there are 25.4 million nanometers in one inch. Viruses come in a wide range of shapes, including circular, cylindrical and stringlike. Some, like the coronavirus, are studded with spikes. These act as docking devices to attach to host cells and then use their unique chemical composition to penetrate the cell membranes.

The sole purpose of a virus is to infect another organism — either an animal, plant or bacterium — make billions of copies of itself and then move on to infect a new host. Small wonder the name for these infectious agents derives from a Latin word that roughly translates to “poisonous slime.”

To achieve its aim, the virion injects its genetic material into the host cell and hijacks that cell’s internal machinery. Instead of doing its designated job, the cell now goes to work replicating the virus. Each virion is so ruthlessly efficient, it can force a cell to make a million copies. The infected cells send out a chemical distress signal in the form of proteins known as cytokines. They set off the inflammatory reaction that causes our immune systems to attack, which results in the symptoms we feel when we’re sick, such as fever, congestion, coughing and sneezing, headache, body aches and gastric distress.

You are correct that antibiotics have no effect on viral illnesses. Antibiotics work by breaching a bacterium’s cell walls, which viruses don’t have, or disrupting its reproduction, which viruses do differently. Due to antibiotic resistance, which is now a serious problem, it’s important not to use these drugs to fight a viral infection. Instead, for viruses such as the flu, your doctor will prescribe an antiviral, which works by interfering when the virus tries to force the cell to make copies of it.