Bartonella henselae, formerly Rochalimæa henselae, is a bacterium that is the causative agent of cat-scratch disease (bartonellosis). Bartonella henselae is a member of the genus Bartonella, one of the most common types of bacteria in the world. It is a facultative intracellular microbe that targets red blood cells. One study showed it invaded the mature blood cells of humans. It infects the host cell by sticking to it using trimeric autotransporter adhesins. In the United States, about 22,000 people(per year?)are diagnosed, most under the age of 20. Most often, it is transmitted from kittens.