The term 'zoonosis' describes an infectious disease that affects humans as well as animals and which can be transmitted between the different species. According to the WHO, a 'zoonosis' is defined as 'any disease or infection that is naturally transmissible from vertebrate animals to humans'.
Different kinds of pathogens can cause zoonoses involving bacteria, viruses, parasites, fungi and other biological units with zoonotic potential (e.g. the agent of Bovine Spongiform Encephalopathy, BSE). The ability of infectious agents to cross the host species barrier is of serious public health concern. More than 200 zoonoses have been described. Some of them are known for a long time and occur sporadically while others re-emerge. The WHO defined an 'emerging zoonosis' as a zoonosis that is 'newly recognized or newly evolved, or that has already occurred previously but shows an increase in incidence or expansion in geographical, host or vector range'. Some of these diseases may 'possibly develop and become transmissible between human beings'.
Important factors in the emergence and spread of zoonotic and vector-borne diseases include the increasing urbanisation and vicinity of human and animal populations going along with changes in social and cultural factors such as food habits, farming practice, international travel and trade (in particular with animals and animal products), which facilitate the global spread of emerging infectious diseases in a short space of time.
The zoonotic pathogens can be transmitted directly between animals and humans or via different kinds of vectors. Arthropods are the most important biotic disease vectors, in particular mosquitoes and ticks and fleas.