Bartonella spp. are emerging bacterial pathogens which continue to achieve an ever-increasing level of clinical importance and awareness.  As Bartonella infections are common zoonoses in the U.S., the existence of Bartonella infection in Sweden, which had not been documented prior to the onset of our investigations, was of considerable interest. Initially, serological parameters of Bartonella infection were evaluated using a series of immunoblot analyses in an effort to elucidate the human humoral immunity induced by this pathogen.  An immunodominant epitope of B. henselae was identified, designated Bh83, which was hypothesized to be a conserved antigen during Bartonella infection. Further, an in-depth characterization of the immunoglobulin (Ig) isotype and IgG subclass response revealed a primary role of IgA and IgG1 in the immune induction during Bartonella infection. As the presence and degree of exposure to Bartonella spp. in Sweden was previously unknown, several studies were initiated to evaluate the level of seroreactivity to Bartonella spp. antigen in various subgroups of the Swedish human and animal population using indirect fluorescent-antibody assay (IFA).  A preliminary study of a select group of patients and blood donors represented the first documentation of Bartonella exposure in Sweden, as well as identified the first two cases of B. quintana-like infections in Sweden.  To assess the endemic level of antibodies to Bartonella spp. in Sweden, a large sampling of blood donor sera collected nationwide were subsequently analyzed for reactivity to six Bartonella strains.  The largest seroprevalence was to B. elizabethae, with relatively low baseline levels of reactivity to the other Bartonella antigens tested.  Correlated risk factors for infection included working outdoors and contact with cats, among others.  Further studies of sera obtained from elite orienteers and intravenous drug users in Sweden revealed a high level of exposure to Bartonella spp. in such cohorts, predominantly to B. elizabethae antigen.  Consistent with the aforementioned findings of elevated titers to B. elizabethae in humans, a high seroprevalence was found in cats, a known reservoir for Bartonella spp., as well. With relatively little known of the pathogenicity and epidemiology of Bartonella infection, these studies provide insight into various aspects of the distribution, risk factors for infection, specific humoral immune response to infection, and the prevalence of Bartonella spp. in human and cat populations in Sweden and serve as a framework for future studies.