This is the map produced by Dr. John Snow during the 1854 outbreak of cholera along London's East End.  By mapping all of the households where cases of cholera occurred, as well asthe pumps from which the population drew water for their daily lives, Snow was able to ascertain that the public pump on Broad Street was contaminated with raw sewage and was thus the reservoir for infection.  When the city removed the pump handle so it could not be used, the epidemic subsided.  This was the first use of descriptive epidemiology, the deliberate gathering of relevant information concerning the impacted population (age, sex, race, diet, socioeconomic status, chain of transmission, etc.) in the attempt to track the specific root cause of disease, as well as the first reported or index case.

Analytical epidemiology involves a thorough investigation of the disease, including data from descriptive studies to determine the etiological agent, its mode of transmission and potential methods of prevention.  These studies are often introspective since they follow a disease outbreak and require examination of infected versus uninfected populations to determine the root causes, predisposing factors and possible routes of disease transmission.

Experimental epidemiology involves hypothesis testing to determine the cause of disease or methods of treating diseases that are already known.  Koch's Postulates can be applied to this type of study when the agent is microbial or modified if the agent is viral or a fastidious intracellular parasite.