The appearance and spread of new parasitic invasions in populations of wild animals are rarely observed. We investigated invasion of the blood-sucking nematode Ashworthius sidemi in European bison from its initial phase, and analysed factors affecting infection severity. The study showed the rapid spread of a new parasite in a bison population (100% prevalence after four years), fast increase in the infection intensity to the mean level of 8,000 nematodes per bison (maximum 44,310), and the strong immune response in animals with severe infection. Factors influencing infection intensity were time since the appearance of the parasite, herd size, bison age and sex. The highest number of parasites was found in sub-adult bison. Adult females were more infected than males. The increase in herd size was followed by an increase in the infection intensity with A. sidemi; the greater when the parasite was present longer in the bison population. Measures aimed at reducing the winter aggregation of bison in supplementary feeding sites reduced the parasitic load. High infection levels led to a decrease in red blood cell parameters and an increase in the level of gamma-globulins. Moreover, increasing intensity of A. sidemi infection significantly increased the probability of occurrence of other parasites. This study allowed a better understanding of the mechanisms of biological invasions in populations of wild animals and the protective immune response of the host. It also showed that adaptive management can effectively lower rates of parasitic infections. The study was financed by the National Science Centre, project No. 2012/07/B/NZ8/00066.