AIDS: Two species of the gram-negative bacilli Bartonella, B. henselae and B. quintana, cause disease in HIV-infected patients. If untreated, infection can be fatal. Manifestations include bacillary angiomatosis (BA), bacillary peliosis hepatis (BP), bacteremia, or a combination of these. BA and BP present as lesions, but bacteremia may be subacute and persist for months without diagnosis. Additionally, patients may acquire cat scratch disease (CSD), but this is more common in immunocompetent patients. BA lesions are usually vascular, friable, and bleed profusely when traumatized. They may be confused with Kaposi's sarcoma (KS), pyogenic granuloma, lymphoma and various subcutaneous tumors and infections. Lesions may affect almost any organ, and appear as angiomatous papules, dry scaling lesions, subcutaneous nodules, cellulitic plaques or deep, highly vascularized, soft tissue masses. Patients may have osseus BA lesions (frequently affecting the long bones); hepatic and/or splenic lesions; bacteremia; or endocarditis. To diagnose infection, lesions should be biopsied and examined. Hematoxylin and eosin staining reveal histopathologic changes; darkly staining organisms are evident after Warthin-Starry silver staining; and electron microscopy allows visualization of the bacillus. An indirect immunofluorescence antibody test (IFA) detects bartonella-specific IgG antibodies. Treatment with erythromycin for at least three months is recommended, or with doxycycline if erythromycin is not well-tolerated. Severely ill patients should receive IV doxycycline with either gentamicin or rifampin for at least four months. To prevent infection, HIV-infected people should avoid traumatic cat contact and exposure to the body louse.