The study of host-pathogen interactions reveals information about disease but also sheds light on fundamental mechanisms of normal cellular function. Using quantitative live cell imaging and other approaches, our research on the bacterial and eukaryotic pathogens, Legionella, Listeria, Salmonella, and Toxoplasma show that these microbes take advantage of the host cell niche by manipulating cellular trafficking pathways. Studies also focus on mechanisms by which intracellular pathogens exploit infected host cells to facilitate cell invasion, acquire nutrients, and regulate differentiation. Infection and other pathological conditions trigger host signaling that result in anti-microbial responses and release of inflammatory mediators. Host regulators of innate immune responses to infection are currently being explored. In addition, chemical signals, such as calcium and NADPH, are being studied in live cells and tissues to determine the effects of biochemical changes on the immune response. Finally, eukaryotic cell cycle regulation and how human disease related proteins are modified during the cell cycle are being investigated.