“When I was young I observed that nine out of ten things I did were failures. So I did ten times more work.”
– George Bernard Shaw –

Epidemiology is the second topic to discuss when considering any disease,and is essential to thoughtful diagnosis and research. Take, for example, the work of Dr John Snow.
John Snow and the Broad Street Pump

The greatest classic in the study of epidemiology concerns the work of Dr John Snow. Snow was the eldest son of a farmer from York, and was introduced to medicine through an apprenticeship with a surgeon during the London cholera epidemic of 1831-2. His experience with cholera left a deep impression, and in 1849, after much thought, he published a small pamphlet suggesting that cholera was a type of poison spread through contaminated food or water. His idea was met with some amusement and not widely accepted. But in 1854 the occasion came to prove his theory, in the form of a new cholera epidemic.
Snow began by plotting on a map the site of deaths from cholera as they occurred. He found that in one particular location, at the corner of Cambridge and Broad Street, more than 500 deaths had occurred from cholera in just 10 days. All the affected households, Snow realised, had one thing in common: they sourced their drinking water from the Broad Street pump. Snow descended on the local Board of Guardians and recommended they immediately put the pump out of action by removing its handle. The Board was incredulous, not a single one believing that Snow was correct. However, they were panic-stricken and carried out Snow’s request. The plague was stayed. Some time later, further evidence appeared for Snow’s insight. An infant girl living at 40 Broad Street had been struck down by cholera and died early in the epidemic. During the diarrhoeal illness, her mother had soaked soiled napkins in a pail and emptied this into the cesspool at the front of the house. An engineer’s examination demonstrated that the cesspool had decayed brickwork and that faecal matter was able to seep into the Broad Street pump water, which lay just three feet away.
When you think or make notes about epidemiology in your Card System you should do so in a clear concise order. Where relevant, you might include such information as: species affected, geographical data, seasonal variation, races affected, age, and gender.

Another important factor in epidemiology is ‘risk factors’ for disease. For some topics you may choose to use risk factors as a subheading on your cards, to make note of those attributes that make patients vulnerable to the disease you are discussing. Under risk factors, you might include such information as environmental factors, family or genetic factors, predisposing medical conditions, or personal traits.