We all know that when we are down with flu, it is a set of microbes that have invaded our bodies, thereby giving rise to the symptoms. In addition to this, we are also aware that boiling water and heating food items kills the germs, and makes them safe for consumption. It isn't rocket science, is it? But way back in the 1800s, it was not this simple. We owe our current understanding of germs or microbes to a French chemist and microbiologist called Louis Pasteur. Apart from his germ theories of fermentation and disease, we also know him as the genius who gave us the process of pasteurization. Given below is a brief account of the germ theory of disease, as well as the significant experiments and observations of Louis Pasteur, his germ theory of fermentation and its extension to diseases. In the 1800s, this idea was not widely accepted, and it took a series of experiments and hard work for Pasteur to prove that air contains infinitely small living organisms, and that these organisms are responsible for diseases. Such tiny living things, which we know today as microorganisms, were first observed by Antonie van Leeuwenhoek, way back in the 1670s. Moreover, the presence of microscopic, disease-carrying agents was proposed by several individuals, hundreds of years before the time of Pasteur. Observations and speculation hinting towards the presence of germs and their involvement in disease also came from the works of Fracastorius, Edward Jenner, Ignaz Semmelweis, Agostino Bassi, John Snow, and several more. But the presence of tiny invisible organisms in air was ridiculed and considered to be a fanciful story. It was only in the 1860s that this proposition was strongly backed by evidence that came from the experiments by Louis Pasteur, followed by pioneering studies by Robert Koch and Joseph Lister. This greatly revolutionized the approach towards the study of infectious diseases as well as the treatment methods. The 1800s witnessed several studies about the generation of life, of which the spontaneous generation theory was much discussed. According to this theory, life originated spontaneously from inanimate objects. Much famous was the recipe for mice, which stated that a fully grown mice can be created from an old cloth and rotting wheat in mere 21 days. Disproving this theory was the first step towards the formulation of germ theory. The papers submitted by Louis Pasteur and Robert Koch regarding their observations and therapeutic suggestion with respect to anthrax, constitute the first formal representations of the germ theory of diseases. But Pasteur's most significant contribution came from his experiments on germs, which permanently ended the debate with spontaneous generation. While demonstrating that boiled water can remain sterile until it is exposed to air, he said: "Life is germ and a germ is life. Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment."