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.Murphy, p.ix Although not the first viruses to be discovered and characterised,Leppard, pp. 4–6 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.Leppard, p. 295 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.Leppard, pp. 268–9 Examples include, rabies, yellow fever and pappataci fever.Leppard, p. 269
