Viruses are smaller and less complex than unicellular microbes, and they only contain one form of nucleic acid—DNA or RNA, never both. Because viruses lack ribosomes, mitochondria, and other organelles, they must rely on their cellular hosts for energy and protein synthesis. They only reproduce within the infected host's cells. The necessity to control viral infections in humans and their domesticated animals prompted the development of animal virology in major part. Viruses, like other infectious agents, enter the body of an animal through one of the animal's surfaces. They then spread systemically by infecting one of the body's surfaces locally or by infecting lymphatic and blood arteries. Animal viruses must identify a unique cellular receptor in their hosts during infection. Host receptor binding is the first step in the virus's life cycle, and it could be a good target for preventing infection. Tissue culture is commonly used in laboratories to create animal viruses.
Here are a few of them: