Rickettsia is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that can be present as cocci (0.1 μm in diameter), rods (1–4 μm long), or thread-like (10 μm long). The term rickettsia, named after Howard Taylor Ricketts, is often used interchangeably for any member of the Rickettsiales. Being obligate intracellular parasites, the Rickettsia survival depends on entry, growth, and replication within the cytoplasm of eukaryotic host cells (typically endothelial cells). Rickettsia cannot live in artificial nutrient environments and is grown either in tissue or embryo cultures; typically, chicken embryos are used: a method developed by Ernest William Goodpasture and his colleagues at Vanderbilt University in the early 1930s.