Most organisms are not pathogenic, particularly the ones in, on and around our body. The body’s resident organisms often prevent pathogens from taking hold by competition for space and by production of natural antibiotics. The body is sealed and where contact with the inner body can occur, the special chemical and cellular defences of the innate immune system are present. The adaptive immune response is powerful and specific in its attack of pathogens. Additionally, it contains memory cells that protect the body from re-infection.
A single infection with a virus results in the astronomical replication of viruses. Although mutation is rare, many mutations will occur in a typical infection, and some of these will produce more contagious viruses. Also, two different virus strains can infect the same host cell to produce a hybrid with a mixture of traits which extend the host range of the viruses. Modern social problems, such as overcrowding, lack of public hygiene, world travel via air, and increased contact between humans and animals, further exacerbate the problem.
This question requires that you think about the tasks that are required for a disease to move from one species to another. These tasks include adaptation and transmissibility. In this case, the measles virus moved from its original reservoir in cattle to humans. It then adapted to humans and is now only seen in this species. Interestingly, it no longer infects cattle. Therefore, there must have been evolutionary pressure that made humans a better reservoir for this virus, which then eventually disappeared in cattle. It is important to remember that in addition to adapting to humans, it is also easily transmitted from human to human.