Why do antibiotics affect bacteria but not viruses?

Bacteria are cells with machinery for maintaining themselves and producing more bacteria. An antibiotic can interfere with part of that machinery. Some antibiotics disrupt construction of a bacterial cell wall. Others interfere with bacterial protein production or other essential processes. Depending on the antibiotic and bacterium, this can kill cells or limit their growth.

The illustration follows one cell-wall target. Many bacteria need a strong wall outside the cell membrane. Blocking the machinery that builds or maintains that wall can make a susceptible bacterium vulnerable. Human cells have membranes but no bacterial cell walls. Target differences help explain selective effects, although antibiotics can still have unwanted effects.

Viruses have a different structure and reproduce using host cells. They do not build bacterial cell walls or possess the same independent bacterial machinery. A medicine that blocks a bacterial wall-building process therefore does not stop a virus through that mechanism. Antibiotics are not treatments for viral infections.

Even a bacterium can resist a particular antibiotic. Some produce enzymes that inactivate the drug; others change a target, reduce drug entry or pump the drug out. Resistant bacteria can survive and reproduce while susceptible ones are removed. Resistance is a property of the microorganisms, and the same antibiotic does not act equally on every bacterial species or strain.