He became associated with St. Mary's Hospital, the youngest of London's teaching hospitals and he remained there for his entire career. He worked with Almoth Wright.
During world war one Fleming was one of Wrights primary assistants to fight many infections and septicemia in British soldier's wounds. Wright and Fleming argued that the Leukocytes of the immune system were the body's most important line of defense against infections. The best treatment for infection, they said, was simply washing with copious saline. Only a few physicians believed this advice. In 1921 Fleming discovered Lysozyme a culture of his own nasal mucus because he had a cold. He accidently sneezed onto a glass culture dish. Fleming soon found out that there was a chemical present in the mucus and that it was a protein.
This was the first of two major chance events in Fleming's career and the broader medical applications of this discovery were unappreciated for years by Fleming and everyone else. One day Fleming seemed to have left a dish lying on the lab bench and when he came back a few spores of an unusual mould had germinated on the plate. When he cultured the bacteria on the plate they grew up to within a few centimetres of the mould, But they were killed and this is how Fleming discovered penicillin. Penicillin has helped lots of people and also saved many lives. Penicillin is still used today to fight infections. Very often when people have chest infections or throat infections they are treated with medicine containing penicillin, which was discovered nearly 100 years ago.