The causes of epidemics are plainly not the pathogens alone as was initially assumed by Koch's school, predisposition and constitution of the population proved to be equally important. Ever since ancient times problems linked with the "constitutio epidemica" have been topical; the "physis", the "natura hominis" and the invironment of man play an increasingly important role in the symptomatology of disease, as can be gathered from such early documents as the "Corpus hippocraticum". Fracastoro distinguished between contagious and non-contagious epidemics. The casual organisms were considered to be miasmas -- noxious emanations -- or "contagia" i.e. likewise toxic substances. Questions concerning the origin of these miasmas turned attention to the environment (air, soil, water) and even led to astrological medicine. Not until the Renaissance were attempts made to differentiate the usual global words for epidemic, such as "loimos", "lues" and "pestis" with the result that a symptom was used more and more to designate a disease. For example, the symptom fever led to the designation "three-day fever" or "four-day fever", "typhus fever". This terminology made a differential diagnosis difficult to establish, thwarted selection measures to check epidemics and the medical world was thus helpless in explaining the causal agents and the phenomena of epidemics. This is illustrated by some epidemiological examples (ergotism, scurvy, yellow fever, English sweat, diphtheria and malaria). In this connection the "morbus novus", the transformation of the pathogen and the change of the pathogen is discussed. Many questions still left unanswered regarding the seasonal incidence, the fluctuation and disappearance of epidemics over decades or even centuries lead more frequently to sociomedical considerations with respect to the victims of epidemics, their predisposition, constitution and environment exposure term "hospital gangrene" with the modern term "hospitalism", we are not dealing with a transformation but a change of the pathogen. The impressive effects produced by antibiotics resulted in carelessness and along with the unprecedented advances in medicine and engineering we forgot to bear in mind that almost all great steps forward have an adverse side. Hygiene and practical medicine have only made a modest beginning in establishing the contact which should indeed be a matter of course in the hospital.