In 1940, World War II in full swing and thousands of soldiers were hospitalized with infections from wounds and illness. That same year, two Oxford University professors changed the course of modern medicine with their discovery of the first antibiotic, penicillin.

Penicillin, available now for 60 years, is still considered the best treatment for numerous types of bacterial infections. But the road to its widespread use as a pharmaceutical treatment was a long one. In the 1870s, scientists had discovered antibiosis, a process that takes place when one kind of organism can kill the microbes of another. Researchers tried to isolate the antibiotic materials from various types of penicillin molds, but the substances were too unreliable.

Scottish bacteriologist Alexander Fleming moved closer to isolating penicillin in 1928, when he noticed that one petri dish of streptococci was contaminated with an airborne penicillin mold that was killing the bacteria it came in contact with. He extracted a yellow fluid from the mold, which he called penicillin (although it was actually a different form of penicillin than he realized), and found it to be effective against a range of germs. Fleming tested the fluid and, not having the skill or resources to isolate in pure form, concluded it wasn’t effective enough. He concluded that penicillin could only have limited use.

Two Oxford professors, Australian pathologist Howard Florey and émigré biochemist Ernst Chain, believed that a pure form of the substance was the key to its effectiveness—and spent the next 12 years working on isolating penicillin.

Sulfa drugs had already been used by doctors, but their side effects often made them too risky. With the outbreak of World War II in 1939, an alternative form of medicine was needed to treat wounded solders. That same year, Florey and Chain received a grant from the Rockefeller Foundation and began researching the therapeutic value of penicillin. Chain and Florey worked to purify the fungus and tested it on numerous strains of bacteria.

In May of 1940, they announced the results of their latest experiments, noting that of eight mice injected with streptococci, only the four that were given penicillin survived and regained their health.

Florey and Chain’s research lab could not keep up with the explosive demand for penicillin, so they traveled to the United States to convince drug companies here to manufacture the antibiotic in large quantities.

By 1944, there was enough penicillin to treat wounded soldiers in North Africa and Europe, as well as civilians who contracted bacterial infections. Fleming was knighted that same year, and in 1945 shared the Nobel Prize in Physiology or Medicine with Chain and Florey.

New antibiotics have since been developed, providing relief to patients who are allergic to penicillin or infected with a bacterial strain that is resistant to the antibiotic—and allow doctors to combine it with other antibiotics to keep down the dosages.