antibiotic
An antibiotic is a drug that fights a bacterial infection by either killing the bacteria or stopping them from multiplying, so your immune system can mop up the rest. Think of an infection as weeds taking over a garden: an antibiotic is the targeted weed-killer that goes after the weeds while trying to leave your own plants alone.
Antibiotics work by exploiting features that bacteria have but human cells do not. Some attack the bacterial cell wall, a structure our cells lack entirely; others jam the bacterial ribosome, which is built differently from ours; still others poison enzymes that bacteria need to copy their DNA. This is the principle of selective toxicity, and it is why an antibiotic that devastates bacteria can be relatively safe for the patient.
Antibiotics only work against bacteria. They do nothing for viral illnesses like the common cold or most sore throats, and taking them when they are not needed drives the spread of resistant bacteria. Using the narrowest effective agent, finishing the prescribed course where appropriate, and reserving antibiotics for true bacterial infections are central to good stewardship.
A patient with a confirmed urinary tract infection is prescribed nitrofurantoin, an antibiotic chosen because it concentrates in urine and targets the likely bacteria.
Antibiotic choice depends on the suspected organism and where the drug reaches in the body.
Strictly, an antibiotic was originally a substance made by one microbe to kill another, while antibacterial covers all such agents including synthetic ones; in everyday clinical use the words are interchangeable.