Bacterial means “of bacteria” or “caused by bacteria.” Bacteria are tiny living things, too small to see, that live on skin, in soil, in food, and inside our bodies. Some help us, like the ones in the gut; some make us ill. So a bacterial infection is an illness those tiny lives started — some sore throats, some food poisoning, some skin problems. Doctors often set this word next to “viral”: a viral cold will not get better with antibiotics, but a bacterial one might. It is a school-and-hospital adjective, a little formal, and it always points back to those tiny living things, not to a feeling or a fashion.