Recent advancements and developments in molecular biotechnology have allowed more precise reclassification of many microorganisms. With the use of these new taxonomy tools, several organisms previously thought to belong to other genera have been recently described as bartonellae. Of the 11 organisms now described as Bartonella spp., only four have been shown to be pathogenic for humans. Table 1 lists the four Bartonella human pathogens along with the their known epidemiology and the scope and range of disease associated with each. All are now considered to be bacteria and can be grown on blood-enriched agar although primary isolation in some may best be achieved in cell tissue culture. B. bacilliformis infection is limited to certain geographic regions in South America where the only human reservoir and the sandfly vector(s) that spreads the disease reside together. Specific antibiotic treatment is dramatically effective in treating the highly fatal, acute intraerythrocytic hemolytic form of the disease, but their effectiveness in treating the vascular proliferative forms (verruga peruana) or the chronic asymptomatic, bacteremic, carrier state of the disease has not been effective. This disease should remain confined to its present endemic geographic areas in South American unless asymptomatic bacteremic persons from these areas migrate to areas where sandflies and humans exist that are capable of establishing this infection in new endemic areas. B. quintana and B. henselae cause a wide range of clinical diseases in humans, the type and extent of which varies significantly with the immune status of the host. In immunocompetent hosts the pathologic response is granulomatous, suppurative, extracellular and intracellular, generally self-limited and usually unresponsive to antibiotic treatment, even to those drugs to which the organism is shown to be sensitive in vitro. In contrast, in immunocompromised hosts the pathologic response is vasculoproliferative, organisms may be seen intracellularly but they are often seen in abundance in extracellular clumps and infection is usually progressive and fatal unless treated. In these patients clinical response to treatment with drugs that are effective in vitro against these organisms has usually been dramatic. Of these agents those that penetrate cells and are found in high concentrations intracellularly, such as erythromycin, clarithromycin, azithromycin, rifampin, doxycycline and gentamicin, appear to be most effective. These agents not only appear to provide the most dramatic treatment response in patients with BA, BP and PRFB and other manifestations of B. henselae (and B. quintana as well) in immunocompromised persons, they appear to be the most promising agents for treatment of persons with both typical and atypical CSD. Further studies will be necessary to more clearly elucidated the mechanisms responsible for the diverse clinical presentations of infection with these organisms in human hosts relative to their immune status. In addition clarification of the epidemiology of B. elizabethae infections in humans may be helpful in understanding the nature of infection with Bartonella organisms.