This question which first came up in France in the mid 1800s is still worth asking today, because one's view of how to obtain optimal health and wellbeing depends upon which side of this debate one agrees with. Germ Theory was put forward most famously by French chemist and microbiologist Louis Pasteur, and states that certain sicknesses are caused by the invasion of micro-organisms that cannot be seen without a microscope. As such, treatment or prevention involves figuring out which microbe (bacteria, virus, fungus, parasite) has invaded the body and then killing it to prevent or stop the disease. This is the theory upon which western medicine is based. You get sick, you go to the doctor and get an antibiotic/antiviral/antifungal to get better. Much of our food, like milk, juices, canned food and even nuts like almonds are pasteurized in order to eliminate the bacteria to avoid sickness. Many people use anti-bacterial soaps and hand sanitizers in order to prevent the spread of infection. These actions are all about killing the germ, and are in support of Germ Theory.