For all of history, a scratch or a chest infection could turn lethal, because nothing could stop bacteria once they took hold inside us. In 1928 Alexander Fleming noticed that a stray mould contaminating one of his culture plates had cleared a halo around itself where the bacteria simply would not grow. The mould, Penicillium, was releasing something that killed them. He called it penicillin. He could not purify it, and the discovery sat for a decade — until an Oxford team led by Howard Florey and Ernst Chain turned it into a medicine. It was the first antibiotic, and it changed what it means to be ill.
Before antibiotics, ordinary infections — a cut, childbirth, pneumonia, surgery — were frequently death sentences. Penicillin opened the door to treating them, and to modern surgery, transplants and chemotherapy, which depend on holding infection at bay.
This is the identical fact set, re-told at a different altitude. Switch any time — the reader keeps your place in the idea, not the prose.