Bartonella is a genus of Gram-negative bacteria. It is the only genus in the family Bartonellaceae. Facultative intracellular parasites, Bartonella species can infect healthy people, but are considered especially important as opportunistic pathogens. Bartonella species are transmitted by vectors such as ticks, fleas, sand flies, and mosquitoes. At least eight Bartonella species or subspecies are known to infect humans. Bartonella henselae is the organism responsible for cat scratch disease.