Alexander Fleming’s discovery of penicillin in 1928, one of the most revolutionary scientific developments of the 20th century, was an act of remarkable fortune. The event was built upon several arbitrary scientific decisions, a freak floating mold spore and an auspicious turn of the weather.

At the time of the greatest scientific hallmark of his career, Fleming was a Scottish research scientist with a thriving practice treating syphilis infections of wealthy London patients. In his spare hours, Fleming worked on his pet project and true passion—the discovery of the proverbial substance that his predecessor, the famed Louis Pasteur, had predicted could kill bacteria.

In 1928, Fleming had built a reputation as a skilled researcher but a disorganized lab technician. His work space was usually in chaos and his bacteria specimen often forgotten or poorly prepared. On Sept. 28, 1928, Fleming returned to his lab after a two-week vacation to check on the results of his original theory—that his own nasal mucus had antibacterial properties. Perusing his samples, he noted that, once again, his staphylococcus cultures had been invaded by a contaminant, this time a mysterious yellow-green mold. Deeming the samples as useless, Fleming discarded them, but later returned to fish them out when a visitor asked to see his work. In that moment, Fleming noted an unexpected development. In his cultures, a clear halo of space where bacteria had not grown surrounded the growths of mold.

Later, Fleming would discover that without his aid or intention, a random spore of rare bacteria-killing mold named Penicillium had drifted from a mycology lab downstairs to his own and taken growth in his cultures. In another stroke of fortune, Fleming had chosen not to incubate the bacteria just as a cold spell hit London, providing just the necessary temperatures for the penicillium to germinate.

At the time, Fleming was still ignorant to the remarkable events behind his discovery, but was able to make the critical connection that the mold inhibited the growth of staphylococcus bacteria. By isolating and extracting a sample of the mold from a piece of moldy bread, Fleming was able to identify the penicillium mold and dub it “penicillum,” which is now known as penicillin. His consequent research proved the mold’s bacteria-slaying effects upon all Gram-positive pathogens, identifying the first antibacterial substance.

The discovery would revolutionize the treatment of bacterial infection, allowing doctors to successfully treat previously untouchable diseases such as syphilis, gangrene and tuberculosis. Named the “wonder drug,” penicillin became the most efficient medicine in the world and spurred the development of an immense pharmaceutical industry dedicated to its creation and proliferation. In 1945, Fleming was rewarded for his work with a Nobel Prize for Medicine.