A number of agents of bacterial infections do not fit the usual categories of gram-positive or gram-negative rods or cocci. This group includes spirochetes and curviform bacteria, obligate intracellular parasites such as rickettsias and chlamydias, and mycoplasmas. This chapter covers not only those agents but also the vectors that carry them—ticks, lice, and other arthropods. The last section of the chapter surveys oral ecology, dental diseases, and the mixed bacterial infections that are responsible for the diseases of the oral cavity.