In my thesis the main goal is to see how the views upon the etiology, treatment and prevention of diphteria has been shaped by the modernization of society and development of modern microbiology during the late 19th century. I use the county of Romsdal as my main focus. In the second part I present the incidence of the disease in the period of 1868-1900, and investigate the regional differences and the hypothesis that the large fisheries in the county was a main route of contagion.As we will see the old ideas of Miasma and Contagionism were different in some crucial points, for example isolation and desinfection. It took time for the revolutionating discoveries of microbiology lead to new understanding of disease amongst the local doctors. The flow of information seem to have gone fast with the spread of diphteriaantitoxin. I conclude that lack of antitoxin can not be the reason why the lethality of the disease stays higher here than in Kristiania towards the end of the period. In the second part we see that the average incidence of diphteria is 1,41 per 1000 inhabitants in areas with a high number of fishermen, and 1,13 per 1000 in areas with few fishermen. I also conclude that the low availability of doctors influence lethality in the district. The different geographical conditions and communicative difficulties affect the registration of disease, and can explain certain geographical variabilities in incidense.