Biological agents can be subdivided into several related groups. These include bacteria and rickettsia, viruses, and toxics. Bacteria and rickettsia are single-celled, microscopic organisms that can cause disease in plants, animals, and humans (Figure 8.42). Some of the diseases caused by bacteria include anthrax, botulism, plague, cholera, diphtheria, tuberculosis, typhoid fever, typhus, Legionnaire’s disease, Lyme disease, and strep infections. Bacterial organisms have a nucleus, intracellular nonmembrane-bound organelles (a specialized cellular part that resembles an organ), and a cell wall. Rickettsia are pleomorphic (come in varying sizes), parasitic microorganisms that live in the cells of the intestines of arthropods (invertebrate animals), such as insects, spiders, and crabs, which have segmented bodies and jointed limbs.