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

Answers from James M. Steckelberg, M.D.

As you might think, bacterial infections are caused by bacteria and viral infections are caused by viruses. Infections caused by bacteria include strep throat, tuberculosis and urinary tract infections. Diseases that result from viruses include chickenpox, AIDS and the common cold.

Bacteria are single-celled microorganisms that thrive in many different types of environments. Some varieties live in extremes of cold or heat, while others make their home in people's intestines, where they help digest food. Most bacteria cause no harm to people.

Viruses are even smaller than bacteria and 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.

Perhaps the most important distinction between bacteria and viruses is that antibiotic drugs usually kill bacteria, but they aren't effective against viruses. In some cases, it may be difficult to determine whether a bacterium or a virus is causing your symptoms. Many ailments — such as pneumonia, meningitis and diarrhea — can be caused by either type of microbe.

Inappropriate use of antibiotics has helped create strains of bacterial disease that are resistant to treatment with different types of antibiotic medications.

Oct. 08, 2011