and southeast Asia. Southeast Asia is especially vulnerable as it has a large number of bat species that generally do not mix, but could easily if climate change forced them to begin migrating. A 2021 study found possible links between climate change and transmission of COVID-19 through bats. The authors suggest that climate-driven changes in the distribution and robustness of bat species harboring coronaviruses may have occurred in eastern Asian hotspots (southern China, Myanmar and Laos), constituting a driver behind the evolution and spread of the virus. Secondary transmission Ebola and Marburg are examples of viral hemorrhagic disease. History During most of human prehistory groups of hunter-gatherers were probably very small. Such groups probably made contact with other such bands only rarely. Such isolation would have caused epidemic diseases to be restricted to any given local population, because propagation and expansion of epidemics depend on frequent contact with other individuals who have not yet developed an adequate immune response. To persist in such a population, a pathogen either had to be a chronic infection, staying present and potentially infectious in the infected host for long periods, or it had to have other additional species as reservoir where it can maintain itself until further susceptible hosts are contacted and infected. In fact, for many "human" diseases, the human is actually better viewed as an accidental or incidental victim and a dead-end host. Examples include rabies, anthrax, tularemia and West Nile virus. Thus, much of human exposure to infectious disease has been zoonotic. Many diseases, even epidemic ones, have zoonotic origin and measles, smallpox, influenza, HIV, and diphtheria are particular examples. Various forms of the common cold and tuberculosis also are adaptations of strains originating in other species. Some experts have suggested that all human viral infections were originally zoonotic. Zoonoses are of interest because they are often previously unrecognized diseases or have increased virulence in populations lacking immunity. The West Nile virus first appeared in the United States in 1999, in the New York City area. Bubonic plague is a zoonotic disease, as are salmonellosis, Rocky Mountain spotted fever, and Lyme disease. A major factor contributing to the appearance of new zoonotic pathogens in human populations is increased contact between humans and wildlife. This can be caused either by encroachment of human activity into wilderness areas or by movement of wild animals into areas of human activity. An example of this