Other animals are those that cause foot-and-mouth disease and cattle plague, swine cholera, infectious anemia and encephalomyelitis in horses, avian plague, psittacosis and the so-called polyhedral diseases of insects, among which the one that affects silkworms is very important, known as "jaundice" because it turns them yellow. Some of these diseases, especially when they appear in epidemic form with great spread, cause economic damage of considerable magnitude. The best-known plant viruses are: that of tobacco mosaic, which causes a characteristic leaf disease in this plant, the various viruses of potatoes, the virus of yellow mosaic of watermelon and that of tomato.
A particular group of viruses is made up of those that live at the expense of bacteria, multiplying inside them until they lyse them, and are therefore called bacteriophages. These are very widespread in nature, in soil, water, feces and especially in materials rich in bacteria. Studied with the electron supermicroscope, bacteriophages appear as small spheres or rods equipped with a slender appendage; their dimensions range from a few tens of mµ to about a hundred mµ.
Similar in many respects to viruses are particular microorganisms called rickettsiae, named in honor of the researcher Ricketts, who died from a laboratory infection while conducting experiments on them. They appear as small rods or spheroidal bodies with dimensions ranging from one-third to 1-2 μ, often characterized by evident polymorphism. The smallest rickettsiae, like viruses, are filterable; the larger ones are instead retained by the filters themselves. They cannot be cultivated in ordinary bacteriological media but, like viruses, must be grown in tissue culture or fertilized eggs. Rickettsiae produce in humans several morbid forms, among which the most serious in terms of symptoms and epidemic spread is epidemic or exanthematic typhus, which presents with high fever and a characteristic hemorrhagic exanthem and is transmitted to humans through the bite of infected lice, which in turn become infected by sucking the blood of sick people. Other rickettsial diseases are Rocky Mountain spotted fever, a distinctly North American disease, Mediterranean eruptive fever, the so-called Q fever, recently observed in Italy, and others. Diseases caused by rickettsiae, unlike those caused by viruses, are quite treatable with antibiotics.