[John Snow, the cholera epidemic and the foundation of modern epidemiology].

John Snow (1813-1858) was a brilliant British physician. Since young he stood out for his acute observation capacity, logical thinking and perseverance, first in anesthetics and later in epidemiology. The successive outbreaks of cholera that affected London, motivated him to study this disease from a populational point of view. He related the appearance of cases to the consumption of "morbid matter", responsible for the acute diarrhea with dehydration that characterizes this disease. Bravely, Snow opposed to certain theories present at his time, sacrificing his own prestige. He was a pioneer in the use of modern epidemiological investigation methodologies such as conducting surveys and spatial epidemiology. Fairly, he is considered nowadays as father of modern epidemiology by the scientific community.