Vaccinations provide an effective way to prevent fatal diseases or reduce their symptoms. This book highlights four infectious diseases: polio, tetanus, measles, and mumps. These four diseases are very harmful to human health and are difficult to treat after the infection because they are caused by a toxin or pathogenic virus. In all four cases, the disease exhibits different modes of transmission and progression of symptoms, which require unique treatment regimens. However, all cases can be prevented by vaccinations administered prior to infection.This book summarizes the symptoms and disease progression of all the four diseases and provides information about the toxin or virus that causes each disease. The ability to harness our immune system through vaccination is discussed in the context of disease prevention. Minor and infrequent risks associated with vaccination are also described, but the very dangerous misconception that vaccines cause autism or other major illnesses is strongly refuted.