Yersinia pestis is a Gram-negative, nonmotile, rod-shaped, coccobacillus bacterium, with no spores. Pathogenesis due to Y. pestis infection of mammalian hosts is due to several factors, including an ability of these bacteria to suppress and avoid normal immune system responses such as phagocytosis and antibody production. Two important antiphagocytic antigens, named F1 (fraction 1) and V or LcrV, are both important for virulence.