By Nick Triggle
BBC News health reporter
Concern about hospital infections such as MRSA is one of the most controversial issues in today's NHS.
Sir Alexander discovered penicillin by accident
About 5,000 people die from such infections out of the many millions who go into hospitals each year.
But 70 years ago, the situation was much worse.
People could often die from a sore throat if the infection spread to the lungs.
And pneumonia and post-operative infections killed one in three of those who got them.
Within a decade that figure had dropped to just a few per cent. The reason - penicillin.
It was the world's first antibiotic when it was discovered by Sir Alexander Fleming, who died 50 years ago on Friday.
The scientist, who was working at London's St Mary's Hospital at the time, stumbled upon the antibiotic partly by accident in 1928.
During some routine research, he noticed a mould had developed on a dish on which he had been growing a bacteria but had forgotten about while he was away on holiday.
What grabbed his attention was that the mould seemed to have destroyed most of the bacteria.
Sir Alexander extracted the antibacterial substance from the mould and penicillin was officially found.
Bacterial infections - effective against the likes of meningitis, pneumonia and blood poisoning.
STIs - Just like the contraceptive pill, penicillin had an impact on the sexual revolution, helping to treat syphilis and gonorrhoea.
War - mass produced by the US to stop soldiers dying form infection wounds towards the end of World War II.
It was another 12 years before the antibiotic was ready for commercial use following tenacious, but often overlooked, work by Oxford University scientists Howard Florey, Norman Heatley and Ernst Chain.
By the end of World War II, the US was mass producing the antibiotic and using it to treat soldiers' war wounds.
And once peace was restored, the public began demanding to be given the so-called "wonder drug".
At the end of the 1940s more than 250,000 patients a month were being prescribed penicillin to treat a variety of diseases from blood poisoning and pneumonia to syphilis and gonorrhoea.
The drug also allowed doctors to carry out increasingly more invasive treatments, which would have been impossible before because of infections.
Kevin Brown, curator of the Alexander Fleming Museum and author of a biography of the scientist, Penicillin Man, said it was one of the most important discoveries in medical history.
"Before Fleming's discovery there really was not treatment for infectious disease. It was a killer," he said.
"It seems strange now, but that was the way it was. When it came onto the market it revolutionised medicine."
Mr Brown said it also brought fame to the scientist, who was born in 1881 in a remote area of Ayrshire in Scotland.
"The public really took Fleming to their hearts. He was unassuming and quiet. It was a case of the little man who had become a real success."
Dr Anne Hardy, from London's Wellcome Trust Centre for the History of Medicine, said Sir Alexander's discovery, for which he was later knighted and won the Nobel Prize, also benefited the whole of medical profession.
"In particular, it transformed public perception of what medicine could do. For the first time people became aware of what doctors could really achieve."
But Dr Hardy said the antibiotic became a victim of its own success as it was soon over-prescribed and resistance started building up.
"All antibiotics are used badly and penicillin was no different. Patients were literally demanding it from their doctors and they got it," she said.
By the mid to late 1940s other antibiotics started coming onto the market to challenge penicillin.
The major two were streptomycin - effective against TB, something beyond even the "wonder drug's" powers - and cepholosporin.
But in 1959, four years after Sir Alexander's death, scientists made a breakthrough in their fight against antibiotic resistance with the first generation semi-synthetic penicillins, which were tailor-made to combat individual diseases such as meningitis.
Other generations followed, but scientists have still struggled to keep pace with the evolution of germs - the hospital superbug MRSA is penicillin-resistant.
Penicillin is still the most widely-used antibiotic
Over recent years, penicillin use has declined by about a quarter - in line with most antibiotics - as doctors have become more cautious about overuse.
And Dr Simon Campbell, president of the Royal Society of Chemistry, believes the pharmaceutical industry is on the brink of losing interest in the penicillin family of antibiotics.
But not everyone shares such a prognosis. Dr Robert Bud, who is researching a history of antibiotics and is principal curator of medicine at the Science Museum, points out that penicillins are still the most widely-used form of antibiotics.
"It is true GPs prescribe it less these days. It used to be common to get if immediately for earache, but doctors will more often wait these days, and sometimes they simply offer a painkiller at first."
However, it is this caution which may ultimately safeguard penicillin for future generations, Mr Bud believes.
"Compared to many other countries such as France, Britain has relatively low levels of penicillin-resistant pneumonia-causing streptococcal bacteria, just a few percent.
"If we are careful I think the use of penicillin will continue."
Half a century after his death, that seems a fitting tribute to one of Britain's most famous scientists.