What is a major "breakthrough" that contributed to a more scientific understanding of disease causation

Answer 1

http://www.historylearningsite.co.uk/louis_pasteur.htm

http://www.bartleby.com/38/7/
In the contents, click on section 2.

Answer 2

What were the major" breakthrough that contributed to more scientific understanding of diesease causation?

Answer 3

What were the major breakthroughs that contributed to a more scientific understanding of disease causation?