Corynebacterium pseudotuberculosis is a gram positive bacteria which causes the equine disease known as pigeon fever. This disease is rapidly spreading, and it is necessary to better understand the immune response to the bacteria in order to develop a vaccine to prevent this disease. Horses typically develop external abscesses, while a smaller percentage develop internal abscesses. Our hypothesis is that the different forms of the disease are determined by the type of TH responses to C. pseudotuberculosis. We are developing a mouse model to study the TH responses to C. pseudotuberculosis and to help vaccine development. Experiments in Fall 2010, tested the effectiveness of different vaccine components of the bacteria. In Spring 2011, we studied the immune response of mice injected with increasing numbers of bacteria. This summer, we continued with a third mouse experiment, which was a continuation of the mouse model begun in Fall 2010. Mouse models serve as excellent tools for studying C. pseudotuberculosis infections since mice are much more convenient and inexpensive than working with horses. In these experiments, I not only observed the immune response to this infection, but also learned how much bacteria is a lethal dose for mice. Once I have determined which vaccine produces the most protective immune response, equine testing of the vaccine will be considered, although this testing cannot be done at Occidental College. Successful development and implementation of a vaccine would positively impact the equine community, as the number of infections caused by C. pseudotuberculosis would be drastically reduced.