Most of human prehistory was spent as small bands of hunter-gatherers; these bands were rarely larger than 150 individuals, and were not in contact with other bands very often. Because of this, epidemic or pandemic diseases, which depend on a constant influx of humans who have not developed an immune response, tended to burn out after their first run through a population. To survive, a biological pathogen had to be a chronic infection, stay alive in the host for long periods, or have a non-human reservoir in which to live while waiting for new hosts to pass by. In fact, for many ‘human’ diseases, the human is actually an accidental victim and a dead-end host. (This is the case with rabies, anthrax, tularemia, West Nile virus, and many others). Thus, much of human development has been in relation to zoonotic, not epidemic, diseases.
Many modern diseases, even epidemic diseases, started out as zoonotic diseases. It is hard to be certain which diseases jumped from other animals to humans, but there is good evidence that measles, smallpox, influenza, HIV, and diphtheria came to us this way. The common cold, and tuberculosis may also have started in other species.
In modern days, zoonoses are of practical interest because they are often previously unrecognized diseases or have increased virulence in populations lacking immunity. The West Nile virus appeared in the United States in 1999 in the New York City area, and moved through the country in the summer of 2002, causing much distress. Bubonic plague is a zoonotic disease, as are salmonella, Rocky Mountain spotted fever, and Lyme disease.
The major factor contributing to the appearance of new zoonotic pathogens in human populations is increased contact between humans and wildlife (Daszak et al.., 2001). This can be caused either by encroachment of human activity into wilderness areas or by movement of wild animals into areas of human activity due to anthropological or environmental disturbances. An example of this is the outbreak of Nipah virus in peninsular Malaysia in 1999, when intensive pig farming intruded into the natural habitat of fruit bats carrying the virus. Unidentified spillover events caused infection of the pig population which acted as an amplifier host, eventually transmitting the virus to farmers and resulting in 105 human deaths (Field et al.., 2001).
Similarly, in recent times avian influenza and West Nile virus have spilled over into human populations probably due to interactions between the carrier host and domestic animals. Highly mobile animals such as bats and birds may present a greater risk of zoonotic transmission than other animals due to the ease with which they can move into areas of human habitation.
Diseases like malaria, schistosomiasis, river blindness, and elephantiasis are not zoonotic, even though they may be transmitted by insects or use intermediate hosts vectors, because they depend on the human host for part of their life-cycle.