Currently there are several branches of epidemiology that can be exemplified as the clinical epidemiology of infectious and non-infectious diseases applying epidemiological issues related to diagnosis, prognosis and therapeutic principles. Methodological and scientific evidence as to epidemics and pandemics today underlie the hypothetical studies that were previously only in the early empirical assumptions given the lack of a more refined knowledge. The article makes an analytical approach as the distribution of diseases over time the historical point of view making a crossing of infectious diseases to chronic diseases in epidemiological transition presents nowadays.