Bacterial infection vs. viral infection: What's the difference?

What's the difference between a bacterial infection and a viral infection?

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Mayo Clinic infectious disease specialist James Steckelberg, M.D., and colleagues answer select questions from readers.

Answer

Simply put, bacterial infections are caused by bacteria and viral infections are caused by viruses. But they're also different in other ways.

Bacteria are single-celled "living" microorganisms that reproduce by dividing. Most bacteria can grow on nonliving surfaces, such as countertops and doorknobs. Not all bacteria are harmful. Some bacteria are even beneficial to your health. But when infectious bacteria enter your body, they can make you sick. Bacteria make toxins that can damage the cells they've invaded. Some bacterial infections, such as strep throat and tuberculosis, are contagious. Others are not.

Unlike bacteria, viruses are not "living" organisms but capsules of genetic material. They require living hosts — such as people, plants or animals — to multiply. Otherwise, they can't survive. When a virus enters your body, it invades some of your cells and takes over the cell machinery, redirecting it to produce the virus. The virus may eventually kill the host cells. Some viral infections, such as influenza and HIV, are contagious. Others are not.

The distinction between bacterial and viral infections is important. Why? Because medications that are effective against one aren't effective against the other.

Bacterial infections are treated with antibacterial antibiotics. Antiviral antibiotics are available for some types of viral infections — but not all. Taking antibacterial antibiotics when you have a viral infection won't treat the viral infection and may even be harmful. Consult your doctor for advice on a specific condition.