Handwashing has not always been believed to reduce the transmission of disease. Up until the late 1800's surgeons didn't scrub or wash their hands before surgery. Often Doctors and medical students would move from disecting a corpse through to examining new mothers without washing their hands. This led to a high mortality rate in maternity wards from puerperal or 'childbed fever'.
In the 1840's an Austrian-Hungarian physician named Ignaz Semmelweis realised the link between hand washing and the spread of infection between patients.
In the Vienna General Hospital, where Dr Semmelweis was a director, patients in the maternity ward were dying at such a rate that they were begging to be sent home rather than stay in the hospital. A colleague died from a dissection wound and Dr Semmelweis decided to analyse the situation. He found that the midwives clinic had a far lower death rate than the students clinic and through trying different hypotheses he concluded that hand washing was the answer. Through insisting the staff wash their hands with various anticeptics Dr Semmelweis greatly reduced the deaths in his wards.
At the time this was met with hostile reaction from other medical professionals who decided that even if there was some truth to the concept, it would take too long to wash their hands between patients. Dr Semmelweis continued his studies and published a book which was once again received poorly. In 1865 he was committed to an assylum where he died shortly after.
After Dr Semmelweis died the germ theory was confirmed by Louis Pasteur, a French Chemist and Microbiologist.
Today hand washing is a part of our everyday life and it is accepted practice to wash our hands before eating, after toileting and sneezing or coughing.