Animal viruses are viruses that infect animals. Viruses infect all cellular life and although viruses infect every animal, plant, fungus and protist species, each has its own specific range of viruses that often infect only that species. Vertebrates The viruses of vertebrates are informally distinguished between those that primarily cause infections of humans and those that infect other animals. The two fields of study are called medical (or clinical) virology and veterinary virology respectively. Although not the first viruses to be discovered and characterised, those that cause infections of humans are the most studied. Different viruses can infect all the organs and tissues of the body and the outcomes range from mild or no symptoms, to life-threatening diseases. Humans cannot be infected by plant or insect viruses, but they are susceptible to infections with viruses from other vertebrates. These are called viral zoonoses or zoonotic infections. Examples include, rabies, yellow fever and pappataci fever. The viruses that infect other vertebrates are related to those of humans and most families of viruses that cause human diseases are represented. They are important pathogens of livestock and cause diseases such as foot-and-mouth disease and bluetongue. Jersey and Guernsey breeds of cattle are particularly susceptible to pox viruses, with symptoms characterised by widespread, unsightly skin lesions. And most people have heard of myxomatosis, which is a fatal pox virus infection of rabbits: once infected they die within twelve days. The virus was deliberately released in Australia in 1950, in an attempt to control the exponentially growing rabbit population. Rabbits had been brought to the continent in 1859 for sport, and having no natural predators, bred at an extraordinary rate. The infection killed 99.8 percent of rabbits, but by the late 1950s, Australian rabbits started to become immune to the virus and the population of rabbits increased, but never to the vast numbers seen before 1950. Companion animals such as cats, dogs, and horses, if not vaccinated, can catch serious viral infections. Canine parvovirus 2 is caused by a small DNA virus, and infections are often fatal in pups. The emergence of the parvovirus in the 1970s was the most significant in the history of infectious diseases. The disease spread rapidly across the world, and thousands of dogs died from the infection. The virus originated in cats, the vector of feline panleukopenia, but a mutation that changed just two amino acids in the viral capsid