Epidemiology is an independent branch of medicine studying the etiology and spread of infectious diseases. Epidemiology didn't exist until John Snow showed up, things were just normal, people getting sick and dying with no trace to why they were suffering from a particular symptom. People believed every illness was as a result of an offense or rebellion against the gods who was supreme, so dying as a result of a particular illness became something they believed they were helpless to since the gods has the final say.
In the year 1854 in East London, there was an outbreak of cholera, people only wondered what the cause was, no doubt they tried assuming factors that might have brought this certain disaster on them but failed in providing answers or solution. They kept recording increase number of people who were infected with similar symptoms each day, but no one knew the cause, they only felt the effect. However there was a single well in the broad street which was their only source of water, everyone who was in that town drank from it but no one saw the sewage around it to be bad.
It was until John Snow came that he traced the source of their illness to be the sewage around the single well that was located in the broad street. People must have been amazed to know that the sewage around the well which was their only source of water was the cause of the cholera and the several death that has occurred. In other to eradicate the effect, they needed to understand the cause. After John Snow's discovery, people were enlightened that it wasn't actually the gods who curses a land with certain disease, but pathogens or man's reckless actions, and epidemiology started from that day.