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Viral epidemics often appear unexpectedly, raising questions about their origins. Some viral epidemics begin with a zoonotic infection, discussed in detail in Chapters 10 and 11. Zoonoses are infections transmitted from other animals to humans. Many viruses that can infect multiple species establish a reservoir in a host in which the virus causes no disease or only nonlethal disease. When a new host is in proximity to an infected reservoir animal, a species jump may occur. While zoonotic transmission may cause disease in the new host, trans-species infection is usually a dead end for the virus. Consequently, zoonotic infections rarely spread from human to human, as is the case for rabies virus, West Nile virus, and avian influenza. Although relatively rare, zoonotic infections are a concern to epidemiologists, because the human host will not have immunity, and the disease that occurs in the new host may be different (often more severe) than that in the reservoir host. The trans-species spread of a human immunodeficiency virus-like ancestor from monkeys to humans is a prime example of zoonotic transmission (Chapter 12), as is the likely zoonosis of SARS-CoV-2 from bats to humans.

Over the past few decades, new, or at least newly discovered, zoonotic and vector-borne viral diseases have emerged, many originating in Southeast Asia and the Western Pacific. These include viruses with which most people have little familiarity, including Japanese encephalitis, Ross River, chikungunya, Nipah, and Hendra viruses. While the first three of these are transmitted by mosquitos, the natural reservoir of both Nipah and Hendra viruses is fruit bats, prevalent in Southeast Asia (Fig. 1.6). As increased contact between animals is the predominant risk factor for trans-species infection, one can envision how changes in the environment or ecosystems of some animals may increase the risk for contact among different species. These changes are of particular concern when humans invade wilderness areas. For example, it is thought that Nipah virus, a paramyxovirus, underwent species-to-species transmission in 1999 in Malaysia, when pig farming began in the habitat occupied by infected fruit bats. The infection spread from bats to pigs and ultimately to the farmers themselves. Hendra and Nipah pose a significant human health threat: in a 2004 outbreak in Bangladesh, Nipah virus killed 60% of the people it infected, and additional out breaks have occurred in almost every year since. Symptoms of infection vary widely: some people experience a transient fever and cough, although complications can include life-threatening inflammation of the brain, respiratory failure, and, following recovery, seizures. Because infection by the virus depends on direct contact with fruit bats or infected hosts, the number of cases is typically low. Nevertheless, as the geographic range of the fruit bat is large, including all of India and much of Southeast Asia, the number of people who could be exposed is enormous. Moreover, it is not necessary to invoke an exotic locale or complex combination of animals for zoonosis to occur; petting zoos, open markets, and state fairs provide sufficient human-animal contact to allow a virus to jump species.

Figure 1.6 Fruit bat geographic range in Southeast Asia, and prevalence of Nipah and Hendra viruses. Outbreaks of Hendra and Nipah virus infections from the late 1990s through 2014 are indicated, as is the geographic range of the Pteropus fruit bat.