Penicillin (macrophotograph below) was the first antibiotic, a substance derived from one micro-organism that will destroy other infectious organisms without harming the host. And in the decades that followed Fleming's breakthrough, nearly 100 more were to be developed.
But even as these powerful new weapons in the war against disease were rolling off the production line, forces were at work that would one day threaten to neutralise them.
By the 1950s, doctors were dishing out these "wonder drugs" for every cough and sniff, even though they were useless against viral infections such as flu or the common cold. Their generosity was encouraged by the manufacturers, and by an increasingly expectant public.
At the same time, farmers discovered that by mixing antibiotics with animal feed, they could keep livestock in cramped conditions without fear of disease, and achieve faster rates of growth.
Unfortunately, the excessive and inappropriate use of antibiotics, especially in countries where they were available without prescription, meant that infectious organisms were being only partly eradicated. And that's when things can turn nasty.
For the bacteria that survive a crude "blanket" attack by antibiotics will tend to be the tougher individuals and by leaving the field open to these survivors we encourage them to develop into increasingly drug-resistant strains. The appearance in recent years of previously unknown and apparently untreatable forms of pneumonia, urinary tract, ear and skin infections and meningitis is now one of the world's most pressing health problems.
It is a result of more than half a century of profligacy by doctors and food producers. While attempts are being made to curb their excesses, many fear that the power of antibiotics, once seen as the greatest discovery in medicine, has already been squandered.