The bacteria of the genus Bartonella are the causative agents for earlier not diagnosed or re-emergent diseases of the humans, danger of which increases in relation with increasing number of persons with the disturbed immune status. Bartonellae are intracellular parasites, the places of their habitation in the humans and animals are the endothelial cells of blood vessels and erythrocytes. The modern data concerning major factors of the Bartonellae virulence and host-bacteria interactions were considered and discussed in this article. The induction of the type IV secretion system, effector protein transmission, inhibition of the endothelial cells apoptosis, and induction of their proliferation lead to formation of new blood vessels and tumors.