Which scientist is best known for formulating the germ theory of disease in the 19th century?

• A. Louis Pasteur
• B. John Snow
• C. Robert Koch
• D. Florence Nightingale

Answer: (A)

The germ theory of disease holds that microorganisms are the cause of many illnesses, not bad air or miasmas. Louis Pasteur provided decisive evidence for this in the 19th century by showing through experiments with sterilized broth and swan-neck flasks that microorganisms from the environment could contaminate and cause growth or decay, while properly sealed or sterilized conditions remained free of microbes. He linked these ideas to disease, arguing that germs invade the body and lead to illness, which laid the groundwork for public health measures like sanitation, sterilization, vaccines, and pasteurization. While others made important contributions—John Snow with epidemiology of cholera, Florence Nightingale with hospital sanitation, and Robert Koch with identifying specific pathogens—the foundational move of formulating and championing germ theory is attributed to Pasteur.