Bartonellosis is a bacterial disorder with a striking geographic distribution: the western slope of the Peruvian, Ecuadorian, and Colombian Andes 2000 to 8000 feet (725 to 2900 meters) above sea level. The causative microorganism, Bartonella bacilliformis, is a small, motile, gram-negative bacillus. Other members of the Bartonella genus causing human disease include Bartonella henselae (cat scratch disease, bacillary angiomatosis, infective endocarditis), Bartonella quintana (trench fever, bacillary angiomatosis, infective endocarditis), Bartonella clarridgeiae (cat scratch disease), Bartonella elizabethae (infective endocarditis), Bartonella vinsonii subsp. berkhoffii (infective endocarditis), Bartonella koehlerae (infective endocarditis), and Bartonella grahamii (neuroretinitis). Bartonellosis, also known as Carrión's disease, is restricted to the Andes region of South America because its vector, a sandfly, Phlebotomus verrucarum, is confined to this geographic area. The clinical manifestations of this unique biphasic illness have been studied extensively. After inoculation by the bite of an infected sandfly, the bacteria enter the endothelial cells of blood vessels and replicate during the incubation period. Within 2 to 6 weeks after infection, the nonimmune host develops Oroya fever (erythrocytic invasive phase), which is characterized by anorexia, headache, malaise, and a potentially striking hemolytic anemia (Figure 124.1). Most commonly, only a few red blood cells are parasitized, and the disease is subclinical or mild without anemia. Less commonly severe disease may occur with up to 100% of erythrocytes parasitized resulting in profound hemolytic anemia and a high mortality.