Epidemiology is the study of the demographics of disease processes, including the study of epidemics and other diseases that are common enough to allow statistical tools to be applied. So, besides contagious diseases, it also focuses on diabetes, coronary heart disease, high blood pressure and the like.
Epidemiology is an important auxiliary branch of medicine, helping to find the causes of diseases and ways of prevention (as in the case of AIDS). It can, using statistical methods such as large-scale population studies, support or refute treatment hypotheses.
Another major use of epidemiology is to identify risk factors for diseases.
Modern medicine, especially evidence based medicine, relies upon 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.History of epidemiology