Some are pathogenic to mammals and man, where they are known to cause the typhus group of fevers. Rickettsia are smaller than bacteria, but larger than viruses. Like the viruses, rickettsia are obligate (they cannot exist on their own or in any other form); they are considered intracellular parasites. Viruses are submicroscopic organisms, smaller than bacteria, and are unable to live on their own. They must invade a host cell and make use of its reproductive mechanism to multiply. Toxins are poisons produced by living organisms, including plants, bacteria, and animals. They would be classified as 6.1 Poisons rather than 6.2 Infectious Substances because they are toxic materials rather than disease-causing agents (Photo 8.9).