The sewage theory of John Snow, a British doctor, opened the history of epidemiological research  In 1854, when there was a massive cholera outbreak in London, John Snow found that most of the deaths had drunk water from the same pump, while people using other pumps or wells were not initially infected with cholera
After investigation, sewage from the sewer contaminated the pump, causing cholera
The factors that exist in every situation where the phenomenon is studied are likely to be the cause of the phenomenon