Bartonella, a close phylogenetic relative of Brucella, are increasingly recognized as zoonotic human pathogens. Many are exceedingly hard to culture and only identified using sensitive molecular diagnostic techniques, leading to their under-appreciation in human disease. Interestingly, a large array of Bartonella species have adapted to infect and colonize mammalians hosts ranging from rodents to bats to cattle. In fact, they appear to be the most ubiquitous cause of chronic, systemic bacterial infection in mammals, approaching infectious prevalences of 90% in some host species. In contrast to Brucella, Bartonella are spread amongst hosts via blood sucking arthropod vectors. The pathogenic strategies that lead to such successful interaction with hosts and vectors is obviously of great interest. We have been exploring Bartonella pathogenic mechanisms using both in vitro and in vivo systems. It is believed that part of Bartonella's pathogenic strategy takes advantage of an unusual and productive interaction with endothelial cells, the cells that line blood vessels, and presumably contribute to the pathogen's ability to sustain chronic bloodstream infection. Indeed, some species in some hosts have the unusual ability to induce angiogenic tumors, a proliferation of capillaries associated with high local levels of bacteria (i.e., bacillary angiomatosis, verruga peruana, and peliosis hepatis).