Epidemiology is the study of diseases in population. For example, a precursor of epidemiology, John Snow, understood that an outbreak of cholera in London was due to infected water :
However, epidemiology is not limited to infectious diseases. For example, the study of type 1 diabetes (T1D) in the human population falls in the field of epidemiology. T1D is an autoimmune disease that affects children and results in the destruction of the insulin-producing beta cells in the pancreas. The treatment for the disease is to inject insulin several times a day for the rest of a patient’s life. The first thing epidemiologists study is the incidence (number of new cases per unit of time) and prevalence (total number of cases in the population) of a disease. For T1D in France, incidence is 13.5 new cases for 100 000 children under 15 per year and prevalence is around 2 out of 1000 people. Incidence after 15 years is not zero but one order of magitude lower.
A second question that epidemiologists are interested in is the causes of the diseases. Genetic causes have been investigated using the genome wide association study design (cf earlier post). Here, I will present the different kinds of study that can be done to try and understand the environmental determinants of a disease. I will start from the study design that provides the weakest evidence and is the less expensive to the study design that provides the strongest evidence but is the most expensive.
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