The discovery by Louis Pasteur that led to the development of vaccines was the principles of germ theory and attenuation of diseases. In the 1880s, Pasteur conducted experiments that demonstrated how weakened forms of pathogens could induce immunity in animals without causing disease. His work on the rabies vaccine and anthrax highlighted the effectiveness of using attenuated microbes to stimulate the immune response. This groundbreaking approach not only paved the way for the development of various vaccines but also revolutionized the field of microbiology and immunology, significantly reducing mortality rates from infectious diseases.