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Honey bees are arguably the world's most iconic pollinator. The presence of honey bees in our landscapes has long invoked images of vitality, diligence, and cooperation. Unfortunately, the current state of honey bee health and productivity paints a rather different picture for this beneficial insect. The survival of honey bees, as well as the livelihoods of those who benefit from their labor (e.g., beekeepers, growers, and ranchers) is under threat from the culmination of a number of detractors to bee health. Exposure to pesticides, poor forage, mite parasites, and pathogens has resulted in heavy annual death of honey bee colonies in the U.S., Europe, and other parts of the world. Among the suspects thought to contribute to bee decline, the fungal pathogen, Nosema ceranae, is found at high prevalence in both healthy and declining/dead colonies. Since N. ceranae is thought to be a novel pathogen of the European honey bee, Apis mellifera, much remains unknown about its pathology at the individual and colony levels, as well as how infection may interact to form synergisms with other factors thought to be responsible for declining honey bee populations. Here, I review the breadth of research conducted on Nosema infection of honey bees in general. I give particular attention to observations on the detection of infection, cytopathology, viability and infectivity of spores, and incidence and geographic distribution. I rely on what is known about a similar and long-recognized pathogen of A. mellifera, N. apis, for context on the host-parasite relationship of Nosema infection of honey bees. As one part of my dissertation research, I applied background information about the effects of infection on the different castes to show how this may disrupt the social structure of a honey bee colony and contribute to its decline. The level of a honey bee colony or even a whole bee are complicated systems to study intracellular microbes such as Nosema that threaten honey bee health and productivity. Continuous cell lines have been lacking to further our understanding of the pathogenicity of Nosema infection in a simplified environment removed from the confounding effects of other factors such as poor nutrition or pesticides. For the second part of my dissertation research, I addressed this deficiency in honey bee research by developing an in vitro system derived from host cells that could be utilized for observing infection in a simplified environment. Whenever possible, I provide suggestions for future research that could broaden our understanding of this pathogen and ultimately improve honey bee health.

In the 80 years since the discovery of viruses as a causative agent of disease in plants, they have continued to be associated with infection of all major taxonomic groups. At present several thousand viruses have been identified, including in man alone. The vast majority of the known viruses infect either plants, vertebrates or bacteria. The first two groups of viruses have been extensively studied because of their economic and medical importance to man. The viruses infecting bacteria have been used extensively in fundamental research in both virology and molecular biology. Comparatively few viruses, at present, are recognised us infecting insects. This is probably due to the neglect of insect pathology in general, rather than to any lack of viruses infecting insects per se. In practice it is not always possible to make a clear-cut distinction between those viruses infecting insects and those viruses infecting plant or vertebrates as some viruses are capable of infecting both.