What is the difference between a virus and bacteria?

Bacteria are living, single-celled organisms with their own machinery for producing energy and reproducing. They are everywhere, and most are harmless or actively beneficial, the trillions in your gut aid digestion and immune function, while a minority cause illnesses like strep throat, urinary tract infections, and tuberculosis. Viruses are fundamentally different and much smaller: essentially genetic material in a protein coat, with no metabolism and no ability to reproduce independently. A virus must invade a living cell and hijack its machinery to make copies of itself, which is why most biologists do not classify viruses as alive. Influenza, colds, COVID, HIV, and measles are viral.

The practical consequence is antibiotics. They work by attacking structures and processes specific to bacteria, cell walls, bacterial ribosomes, so they do nothing whatsoever against viruses, which lack those targets. Taking antibiotics for a cold or flu provides no benefit, carries side effects, and contributes to antibiotic resistance, one of the more serious slow-moving public health threats, in which overuse selects for bacteria the drugs no longer kill. Viral illnesses are treated with antivirals where they exist, which are far fewer and more specific, or with supportive care while your immune system clears the infection, and are prevented most effectively by vaccines. Since both can cause fever, fatigue, and aches, telling them apart often requires testing rather than symptoms, which is exactly why a clinician declining to prescribe antibiotics for your cold is practicing good medicine rather than withholding care.