Only a few bacteria are harmful. The good bacteria keep the world clean and green. They release nitrogen into plants. Bacteria in our mouth keep harmful bacteria away. Our white cells make macrophages that are suicide cells destroy bad bacteria and then they die.
Antibiotics kill good and bad bacteria.
The lactobacillus in the vagina is destroyed causing yeast overgrowth, and Eshcericia coli in the bowel is killed with antibiotics causing diarrhea. Deadly bacteria reproduce in less than one hour, doubling all the time. As they reproduce they are not the same bacteria, but mutants occur. The antibiotics may wipe them away anyway, but some resist and may survive in the host. Previously harmless bacteria now become hostile.