European Foulbrood (EFB) is a bacterial brood disease of honey bees (Apis mellifera), caused by Melissococcus plutonius. In recent years, a significant increase in EFB cases has been observed in Switzerland and also other countries, and this situation threatens honeybee health, causes damages to apiculture, and requires extensive measures to control the spread of the disease. In this thesis, I study the virulence of 16 M. plutonius isolates, on the basis of in vitro infection assays (chapter I). The virulence levels of individual isolates varied greatly and were correlated with the growth dynamics of an isolate. Three of the tested M. plutonius isolates contained the gene mtxA, which codes for the putative toxin melissotoxin A. Mortality rates in honey bee larvae after experimental infection with one of these three isolates were significantly higher than after infection with isolates lacking this gene, suggesting mtxA to increase virulence of M. plutonius. In chapter II, I investigate the genetic diversity and spatial distribution of Swiss M. plutonius, by applying multi locus sequence typing (MLST) to 160 isolates sampled in two periods (2006-07 and 2013). I also analyze the presence of mtxA. Five novel sequence types (ST) have been only identified in Swiss M. plutonius samples, and mtxA was detected more often in second sampling period isolates. In search of new solutions to control EFB in chapter III, I collect honey bee queens from apiaries with acute EFB outbreaks: queens from colonies being severely affected by EFB (EFB+), and queens from colonies showing no symptoms of the disease (EFB-). I infect the larvae of these queens in vitro with M. plutonius strain CH 49.3 or with strain CH 40.2. In parallel, I test the colonies established by these queens for hygienic behavior, by freeze-killed brood bioassays. Larvae of EFBqueens die off faster after infection with the lower virulence M. plutonius strain CH 40.2, than larvae of EFB+ queens. No such difference was observed after infection with high virulence isolate CH 49.3. Colonies headed by EFB- queens showed higher levels of hygienic behavior than colonies founded by EFB+ queens. EFB- colonies could therefore have developed a strategy to keep M. plutonius levels low within their colonies, consisting of earlier death of infected larvae, combined with elevated hygienic behavior. Overall, this thesis expands our knowledge of EFB, by providing new insights into the virulence and genetic diversity of the pathogen M. plutonius, as well as on the disease resistance mechanism of honey bees.