Every antibiotic covers a spectrum. That is the range of microbes it can kill. A narrow drug hits a small slice. A broad drug hits many kinds at once. Here is the catch. The same drug that clears the infection also mows down the body's protective normal flora. Pick an infection, then pick a drug, and watch both sides of the trade.
The gut flora and the infectioneach shape is one colony
The ringed coral colonies are the infection. Everything else is normal flora. A faded, crossed colony has been killed. Yeast is never killed by an antibacterial drug, so it swells as its competitors go.