Horizontal transmission rates of Paenibacillus larvae within and between colonies in relation to spore source or distance to infected colonies were determined. Bacterial spores found on adult bees represent a greater threat to larvae than spores found in honey. Furthermore, different methods for selecting disease tolerant (hygienic) stock were evaluated demonstrating large differences in brood removal rates that depend on the method used. We quantified for the first time the rate of horizontal transmission of Varroa destructor at an isolated study site. We confirmed that robbing is the crucial factor for the distribution of mites between colonies. However, only a small proportion of the mites from a collapsing colony can be found in neighbouring colonies; the major proportion of the Varroa population "disappears". A lower susceptibility to V. destructor of colonies headed by queens derived from an 8 year selection experiment on the island of Gotland (Sweden) could be confirmed. We found significant differences between "Gotland colonies" and non-selected control colonies with respect to the population dynamics of bee brood and Varroa mites. Interesting features of the Gotland host colonies were a shorter life-span of the adult bees and a lower brood-to-bee ratio of mite infestation. We established RT-qPCR protocols for the quantification of ten honeybee viruses and numerous DWV strains in individual bees, normalized using two honeybee internal control mRNAs. We also sequenced and characterized SBPV, though across Europe we found SBPV to be naturally very rare. Mechanisms of virus transmission and virus tolerance were studied by following diverse infection routes of viruses among bees within the colony and within experimental groups of honeybees. We experimentally demonstrate the transmission of DWV from adult workers to larvae and among adult worker honeybees, the latter through trophallactic exchange of food. Varroa destructor mites also mediate DWV transmission, leading to considerably higher viral loads than transmission without mites (e.g. oral transmission) and mite-mediated transmission seems to be the most important route for epidemic DWV transmission in honeybees. We also demonstrated venereal and vertical transmission of DWV, a novel component of between-colony transmission. Yet DWV infection did not seem to have any effect on the ability of drones to reach mating sites (drone congregation areas). We found surprisingly little variance between honeybee races in viral disease susceptibility but large between-colony differences in virus replication in all races. Using two naturally coexisting DWV variants, we did not detect differential transmission of these variants via the venereal-vertical transmission route, during trophallactic exchange of food, adult-larval feeding or through feeding by V. destructor. © 2011 Nova Science Publishers, Inc.