Bacteria belonging to the Bartonella genus are extremely fastidious, slow-growing gram-negative rods that are dependent on blood or hemin for growth. In 1992, the Bartonella genus was a single species. Bartonella henselae,  quintana,  vinsonii and  elizabethae were previously classified in the genus Rochalimaea and considered rickettsiae, but were moved to the genus Bartonella in 1993. Currently, there are twenty-nine officially recognized species of Bartonella, and at least six additional Bartonella species for which isolates exist, but that have not yet been officially recognized ( phoceens,  ratti massiliensis,  tamiae,  washoensis,  australis, and  rochalimae). Eight Bartonella species have been isolated from humans:  henselae,  quintana,  elizabethae,  bacilliformis,  rochalimae,  washoensis,  tamiae, and  vinsonii subsp. arupensis. Of these,  quintana,  bacilliformis, and  henselae have been isolated most frequently from humans. The remaining five species,  elizabethae, rochalimae,  washoensis,  tamiae and  vinsonii subsp. arupensis have been isolated only from three or fewer humans. AdditionalBartonella species have been associated with human infection by DNA amplification from human tissue and fluids.

Contemporary Bartonella infections were first identified in the US in 1983, early in the AIDS epidemic, when immunosuppressed HIV-infected patients developed striking cutaneous and hepatosplenic vascular lesions. These lesions were named bacillary angiomatosis (in skin and lymph nodes), and bacillary peliosis in liver and spleen (see Figures). The causative organisms remained unknown until bacterial DNA was amplified and cultured from human blood and tissues. Initial epidemiological investigation revealed that patients who developed bacillary angiomatosis lesions were statistically more likely to have had contact with cats. A more detailed molecular epidemiology study determined that although bacillary angiomatosis lesions caused by  henselae were associated with cat owners, the other agent of bacillary angiomatosis, quintana, was associated with homeless patients who had exposure to body lice. Bartonella henselae was subsequently identified as the agent of cat scratch disease in immunocompetent patients, and the domestic cat as the reservoir and vector for transmission of the bacterium to human. The prevalence of  henselae bacteremia in domestic cats is surprisingly high, e.g., 41% in the San Francisco Bay area. Of note, although 35 Bartonella species have been identified, only  quintana and  henselae have been isolated from bacillary angiomatosis lesions of HIV-infected patients.

Bartonella alternate between two niches: the gut of obligately hematophagous arthropod vectors and the bloodstream of the mammalian reservoir .Bartonella species usually have both arthropod and mammalian host specificity, e.g., the domestic cat is the mammalian host for  henselae and the cat flea is the arthropod vector that infects cats.  Humans are believed to be the definitive reservoir for quintana and  bacilliformis, and the body louse and sand fly, respectively, are their arthropod vectors. Indeed, the specificity between a Bartonella species and its vector is illustrated by  bacilliformis, which infects humans only in the restricted geographic region where the transmitting sand fly vector is found: in the South American Andes mountains. It is now evident that numerous mammalian species have persistent bloodstream infection with their own cognate Bartonella species. Although  quintana,  henselae, and  bacilliformishave been isolated repeatedly from humans, the association of other Bartonella species with human infection often reflects the unique exposure of those individuals to the mammalian host and/or its arthropod vector: e.g., a woman who developed  alsatica lymphadenitis following rabbit exposure; another woman diagnosed with rochalimae bacteremia after receiving insect bites while traveling in Peru; a cattle rancher with  vinsonii subsp. arupensis bacteremia; and a person living in rural Nevada who developed  washoensis infection.