Alexander Fleming was born at Lochfield farm near Darvel, Ayrshire, Scotland. He was the seventh of eight children in a farming family. His early life was spent in the rural countryside, fostering a keen observation of nature, a trait that would later contribute to his scientific breakthroughs.

Following local schooling, Fleming moved to London at the age of 14, where he lived with his older brother, Tom. He attended Regent Street Polytechnic, developing foundational knowledge in various subjects and preparing him for his future medical studies.

Fleming enrolled at St. Mary's Hospital Medical School in London, where he excelled in his studies and developed an interest in bacteriology. Initially he intended to be a surgeon, but a chance encounter changed his path. He qualified with an MBBS degree in 1906, marking the beginning of his professional medical career.

During World War I, Fleming served as a captain in the Royal Army Medical Corps and worked in military hospitals on the Western Front. This experience exposed him to the devastating effects of bacterial infections in wounds and fueled his desire to find more effective treatments than existing antiseptics, which often did more harm than good.

While working at St. Mary's Hospital, Fleming discovered lysozyme, an enzyme present in body fluids like tears and saliva that has antibacterial properties. Though not effective against most pathogenic bacteria, this discovery demonstrated his keen observational skills and laid the groundwork for his later, more impactful finding.

Fleming's most significant breakthrough occurred when he returned from vacation to find a mold contaminating a petri dish containing Staphylococcus bacteria. He observed that the mold, later identified as "Penicillium notatum", had inhibited the growth of the bacteria around it. This serendipitous observation led to the discovery of penicillin, the first antibiotic.

Despite recognizing the potential of penicillin, Fleming faced challenges in isolating and purifying it in sufficient quantities for clinical use. He published his findings in 1929, but it was not until the late 1930s and early 1940s that Howard Florey, Ernst Chain, and others at Oxford University successfully isolated and purified penicillin, paving the way for its mass production and clinical application.

Fleming, along with Howard Florey and Ernst Chain, was awarded the Nobel Prize in Physiology or Medicine in 1945 for the discovery of penicillin and its curative effect in various infectious diseases. This prestigious award recognized the monumental impact of their work on medicine and solidified Fleming's place in scientific history.

Following the Nobel Prize, Fleming continued his research and lectured extensively, promoting the responsible use of antibiotics to prevent the development of resistance. He received numerous honorary degrees and awards, becoming a symbol of scientific innovation and contributing to the ongoing fight against infectious diseases.

Alexander Fleming died in London at the age of 73 from a heart attack. His ashes were interred in St Paul's Cathedral, a testament to his profound contribution to humanity and the enduring legacy of his discovery, which continues to save countless lives around the world.