As a group of faculty with expertise and research programs in the area of host-pathogen interactions (HPI), we are concentrating on students' learning of HPI concepts. As such we developed a concept inventory to measure level of understanding relative to HPI after the completion of a set of microbiology courses (presently eight courses). Concept inventories have been useful tools for assessing student learning, and our interest was to develop such a tool to measure student learning progression in our microbiology courses. Our teaching goal was to create bridges between our courses which would eliminate excessive overlap in our offerings and support a model where concepts and ideas introduced in one course would become the foundation for concept development in successive courses. We developed our HPI concept inventory in several phases. The final product was an 18-question, multiple-choice concept inventory. In fall 2006 and spring 2007 we administered the 18-question concept inventory in six of our courses. We collected pre- and postcourse surveys from 477 students. We found that students taking pretests in the advanced courses retained the level of understanding gained in the general microbiology prerequisite course. Also, in two of our courses there was significant improvement on the scores from pretest to posttest. As we move forward, we will concentrate on exploring the range of HPI concepts addressed in each course and determine and/or create effective methods for meaningful student learning of HPI aspects of microbiology.