What’s the difference between bacteria and viruses?

I was recently having an appropriately socially distant conversation with my friend Nancy, a bright and educated professional whom I’ve known for years. She was telling me that until a recent conversation with a biologist friend, she hadn’t really understood the difference between bacteria and viruses. That got me thinking she’s probably not alone. Bacteria and viruses are too small to see without a microscope. Both can cause disease, and that’s about where the similarities end.

Bacteria are single-celled organisms that are the most abundant form of life on Earth. Most are harmless. They have a cell membrane and usually a rigid cell wall. Inside, they have their genetic code in DNA like we do, and the subcellular parts that it takes to consume nutrients, live, grow and reproduce. They live nearly everywhere on Earth – in soils, streams, deep oceans, hot springs and in and on us.

Given the right temperature and nutrients, most will grow independently in nature or the laboratory. Most of them that live on and in us live in partnership with us, providing defense against disease-causing bacteria, producing nutrients from foods that we can’t digest on our own and helping our immune and other systems to function properly. A few of them can cause disease. Staph and strep bacteria are examples of bacteria-causing disease.

Viruses, on the other hand, are strange creatures. They need a host cell to live and reproduce and cannot survive alone. They are usually submicroscopic particles made of a protein shell containing genetic materials (DNA or RNA) that are the plans for their components. When they come into contact with the kind of cell they are capable of infecting, they gain entry into that cell, say in your nose, lungs or tomato plant, and inject their genetic material into the host cell.

They are then able to take over parts of the inner structure of the host cell to build proteins and genetic copies of themselves, which then self-assemble and leave the cell. Usually this results in the death of the host cell, but with some viruses the host cell can change and become cancerous. For example, human papillomavirus, or HPV, can lead to cervical cancer. Measles, mumps, chicken pox and the common cold are other viruses.

One way I think about the difference is to compare bacteria to an animal that can live, eat, grow and reproduce. A virus can be compared to a virus file on a flash drive that is completely inert until you plug it into a computer, its host, and download it. If it’s a program that can run on its own in the computer host, and it’s not something you are installing on purpose, it’s a computer virus. The similarities between real and computer viruses are pretty remarkable.

Antibiotics that have been around for the last 100 years or so can kill or stop the growth of bacteria. Because bacteria and humans have similar subcellular processes, antibiotics can have toxicity for us since the ways they disrupt bacterial growth also can affect us. And because bacteria adapt resistance to antibiotics, there is reason to be conservative when we use antibiotics.

There are fewer antiviral drugs because viruses commandeer our cellular processes, and disrupting them without injuring the host (you and me) is more complicated. The difference between bacterial and viral illnesses is why traditional antibiotics are not used to fight viral infections. The challenge for us in health care, as well as our patients, is being able to tell the difference when symptoms can be so similar.