Epidemiology is the study of the demographics of disease processes, and includes, but is not limited to, the study of epidemics. It is an important auxiliary branch of medicine, helping to find the causes of diseases and ways of prevention. See AIDS.
It can, using statistical methods, prove or refute treatment hypotheses. See large scale population studies.
Modern medicine and especially evidence based medicine vitally needs sound epidemiological methods.
Dr. John Snow is famous for the suppression of an 1854 outbreak of cholera in London's Soho district. He identified the cause of the outbreak as the public water pump in Broad Street, and sabotaged it, thus ending the outbreak.
This was a major event in the history of public health, and can be regarded as the founding event of the science of epidemiology.