In the 1840s the Vienna General Hospital was one of the great centres of European medicine, yet many mothers died of ' childbed fever.' Ignaz Semmelweis set out to find the cause of this disease.

In the small French town of Arbois stand the graves of Louis Pasteur 's three young daughters. Two had died from typhoid, one from an inoperable tumour-a stark reminder of the background to his struggle to find the cause of disease.

The early 1870s. Although a revolution is under way in the understanding of disease, ignorance and opposition to the new ideas exist at all levels. Can Pasteur or Koch find a practical outcome of their experiments that will convince the sceptics?