Nosema bombi is an obligate intracellular parasite that infects different bumblebee species at a substantial, though variable, rate. To date its pathology and impact on host fitness are not well understood. We performed a laboratory experiment investigating the pathology and fitness effects of this parasite on the bumblebee Bombus terrestris. We experimentally infected one group of colonies with N. bombi spores at the start of the worker production, while a second uninfected group of colonies served as controls. During colony development we collected live workers for dissections to measure infection intensities. In parallel, we measured several life history traits, to investigate costs to the host. We succeeded in infecting 11 of 16 experimental colonies. When infection occurred at an early stage of colony development, virtually all individuals were infected, with spores being found in a number of tissues, and the functional fitness of males and young queens was reduced to zero. Further, the survival of workers from infected colonies and infected males were reduced. With such severe effects, N. bombi appears to decrease its opportunities for transmission to the next host generation.