General Information: This strain is an egg-passaged attentuated avirulent strain that has been used as a live vaccine and was originally isolated from a typhus patient in Madrid. This species is the causal agent of typhus. The bacteria are transmitted via an insect, usually a tick, to a host organism, in this case humans, where they target endothelial cells and sometimes macrophages. They attach via an adhesin, rickettsial outer membrane protein A, and are internalized where they persist as cytoplasmically free organisms. Transovarial transmission (from mother to offspring) occurs in the invertebrate host.