Epidemiology is the research discipline, and deals with naturally occurring phenomena in the field, rather than under the controlled laboratory conditions. Epidemiology is essential tools for public health officials and policy makers, and also the basic tools for biomedical researchers. Epidemiology is not a magic wand to control human or animal diseases, but a compilation of common sense. It is easy to understand, but easy to misuse. Step-by-step considerations with common sense is required to prevent fallacies.