Zoonoses are infectious diseases of animals (usually vertebrates), that can naturally be transmitted to humans.
Major modern diseases such as Ebola virus disease and influenza are zoonoses. Zoonoses can be caused by a range of disease pathogens such as viruses, bacteria, fungi and parasites. Most human diseases originated in animals, however, only diseases that routinely involve animal to human transmission, like rabies, are considered as zoonoses.
Ebola is suspected of being bat-borne, though that has yet to be proved beyond doubt. Bats also look like the origin of MERS, a viral illness that appeared in 2012 in the Middle East, and SARS, another virus, which burst upon the world from southern China at the end of 2002. HIV, meanwhile, came from other primates. The pandemic version, HIV1, was once a chimpanzee virus. HIV2, largely restricted to west Africa, came from a monkey
Zoonoses have different modes of transmission. In direct zoonosis the disease is directly transmitted from animals to humans through media such as air (influenza) or through bites and saliva (Rabies). In contrast, transmission can also occur via an intermediate species (referred to as a vector), which carry the disease pathogen without getting infected. When humans infect other animals; it is called reverse zoonosis or anthroponosis.
Zoonoses are particularly likely to develop when people and animals live in close proximity to each other. One reason southern China often spawns them (SARS was not unique; a lot of influenza begins there, too) is that the region has a plethora of small farms, in which many species of animal live in close quarters with each other and with human beings. The constant crossing of pathogens between the species involved makes it more likely that one will emerge that can thrive in people. Agriculture is not the only sort of proximity that can foster zoonotic disease. HIV1 is suspected to have started with a hunter who killed a chimpanzee in the forest. In this context, the extensive clearance of forests, at present a serious environmental issue in many poor countries, brings people into habitats they might previously not have visited. That, in turn, is suspected by some to be increasing the amount of zoonotic disease.
All this suggests that disease-surveillance, which currently concentrates on people, needs to be expanded to look at animals as well. It is necessary to develop a network of investigators in tropical countries who are watching for signs of crossover by monitoring both animals and people.* Inspiration from The Economist, latest copy.