Which discovery in the 1800s lead to the development of germ theory

Discovery of vaccine for smallpox, viruses and actual organisms that cause many diseases lead to the development of Germ Theory.

Explanation:

Theory was the 'predominant theory' of 'disease transmission' before germ theory but it is no longer accepted as a 'scientific theory' of diseases. 'Formal and reliable experiments' on germs and diseases relationship were 'conducted by Louis Pasteur'. He showed that growth of microorganisms was not a spontaneous generation and his pasteurization experiment provided key pieces of evidence that supported germ theory.