Alexander Fleming was born in a remote, rural part of Scotland. The seventh of eight siblings and half-siblings, his family worked an 800-acre farm a mile from the nearest house. The Fleming children spent much of their of time ranging through the streams, valleys, and moors of the countryside. "We unconsciously learned a great deal from nature," said Fleming.

When their father died, Fleming's eldest brother inherited the running of the farm. Another brother Tom had studied medicine and was opening a practice in London. Soon, four Fleming brothers and a sister were living together in London. Alec, as he was called, had moved to London when he was about 14, and went to the Polytechnic School in Regent Street. Tom encouraged him to enter business. After completing school he was employed by a shipping firm, though he didn't much like it. In 1900, when the Boer War broke out between the United Kingdom and its colonies in southern Africa, Alec and two brothers joined a Scottish regiment. This turned out to be as much a sporting club as anything; they honed their shooting, swimming, and even water polo skills, but never went to the Transvaal. Soon after this, the Flemings' uncle died and left them each 250 pounds. Tom's medical practice was now thriving and he encouraged Alec to put his legacy toward the study of medicine.

Fleming took top scores in the qualifying examinations, and had his choice of medical schools. He lived equally close to three different schools, and knowing little about them, chose St. Mary's because he had played water polo against them. In 1905 he found himself specializing as a surgeon for almost as random a reason. His switch to bacteriology was even more surprising: if he took a position as a surgeon, he would have to leave St. Mary's. The captain of St. Mary's rifle club knew that and was desperate to improve his team. Knowing that Fleming was a great shot he did all he could to keep him at St. Mary's. He worked in the Inoculation Service and he convinced Fleming to join his department in order to work with its brilliant director -- and to join the rifle club. Fleming would stay at St. Mary's for the rest of his career.

In 1909 German chemist-physician Paul Ehrlich developed a chemical treatment for syphilis. He had tried hundreds of compounds, and the six hundred and sixth worked. It was named salvarsan (meaning "that which saves by arsenic"). The only previous treatments for this disease had been so toxic as to often kill the patient. Ehrlich brought news of his treatment to London, where Fleming became one very few physicians to administer salvarsan. He did so with the new and difficult technique of intravenous injection. He soon developed such a busy practice he got the nickname "Private 606."