Zoonotic diseases: classification, global burden and the one health imperative

Wildlife and arthropod vectors are the twin engines that drive much of the world’s zoonotic disease. Wild animals serve as the natural reservoirs for the majority of the pathogens that have emerged in people in recent decades, while blood-feeding vectors such as mosquitoes, ticks, fleas and sandflies provide the bridge that carries many of these agents from animals to humans. This review examines both halves of the equation and, crucially, their intersection. We consider why certain wildlife taxa—bats, rodents, primates and birds in particular—are such prolific sources of pathogens, and how human encroachment on natural habitats converts a latent reservoir into an active threat. We then survey the principal groups of arthropod vectors and the diseases they transmit, noting that vector-borne diseases account for more than seventeen per cent of all infectious diseases and over seven hundred thousand deaths each year. The two themes converge in the many vector-borne diseases that are maintained in sylvatic cycles among wild hosts—Kyasanur Forest disease, West Nile fever, plague and the leishmaniases among them—and that spill into people when human, vector and reservoir overlap. We review representative mosquito-borne, tick-borne and other vector-borne diseases, with attention to the Indian context, before discussing the ecological drivers of emergence, the principles of surveillance and control, and the One Health framework within which veterinarians play an indispensable role at the wildlife–vector–human interface.